Message Sent To Deallocated Instantce - ios

Question

I'm using game center to send data between two players. For some reason I keep getting a deallocated instance message. Here's the code:
- (void)sendGameMove:(uint32_t)i andj:(NSString *)j {
MessageMove message;
message.message.messageType = kMessageTypeMove;
message.i = 1;
message.j = #"Testing 1 2 3s";
NSData *data = [NSData dataWithBytes:&message length:sizeof(MessageMove)];
MessageMove * messageMove = (MessageMove *) [data bytes];
NSLog(#"Message I: %i", messageMove->i);
NSLog(#"Message J: %#", messageMove->j);
[self sendData:data];
}
(I Filled in the i and j arguments for what i'm passing). In this method the NSLog statements both log what they're supposed to after creating the NSData object but when I sent that NSData object to the method [self sendData:data]:
- (void)sendData:(NSData *)data {
MessageMove * messageMove = (MessageMove *) [data bytes];
NSLog(#"Message I: %i", messageMove->i);
NSLog(#"Message J: %#", messageMove->j);
NSError *error;
BOOL success = [[GCHelper sharedInstance].match sendDataToAllPlayers:data withDataMode:GKMatchSendDataReliable error:&error];
if (!success) {
[self matchEnded];
}
}
In the NSLog statement the first one works fine so I get:
"Message I: 1"
in the console but for the second log statement I get :
"*** -[ respondsToSelector:]: message sent to deallocated instance"
the code to break down the data object is the exact same in the second method as in the first. Any ideas?

Answer 1

I'm guessing that MessageMove is a struct like:
typedef struct {
int i;
NSString *j;
} MessageMove;
The problem is that you're sending only the contents of the struct itself. In memory and on the network, it'd look something like this:
01000000 07FCA700
-----------------
i j
When the second device receives the message and tries to read the j pointer, it crashes because there's nothing there: that pointer was only valid on the origin device. The struct didn't even contain the contents of the string at all.
To fix this, you need to actually send the string in the message. Flexible array members are one way of storing a string directly in the struct:
typedef struct {
int32_t i; // explicit integer sizes are a good idea for network protocols
int32_t j_length;
char j[]; // the flexible array member
} MessageMove;
Send:
NSString *s = #"Testing 1 2 3s";
NSData *d = [s dataUsingEncoding:NSUTF8StringEncoding];
size_t messageSize = sizeof(MessageMove) + [d length];
MessageMove *mm = calloc(1, messageSize);
mm->i = 1;
mm->j_length = [d length];
memcpy(mm->j, [d bytes], [d length]);
[self sendData:[NSData dataWithBytes:mm length:messageSize]];
Receive:
MessageMove *mm = (MessageMove *)[data bytes];
assert(mm->j_length == [data length] - offsetof(MessageMove, j));
NSString *j = [[NSString alloc] initWithBytes:mm->j length:mm->j_length encoding:NSUTF8StringEncoding];
The assertion prevents reading past the end of the data buffer. offsetof is a macro from <stddef.h>.
For anything more complicated than this, I'd recommend serializing to and sending plist or JSON instead. They handle all the string copying details for you and let you send arrays and dictionary too.