How to store struct information on a uint64_t for Game Center challenges - objective-c

Question

I'm trying to store some game related information on the uint64_t context property of GKScore - to create a better gaming experience with the new Game Center Challenges. However, I'm not getting it right. I built a struct like below:
typedef struct{
unsigned int gameMode;
unsigned int destroyed;
unsigned int duration;
} GameInfo;
I try the following:
uint64_t myContext
GameInfo info;
info.gameMode = 2;
info.destroyed = 50;
info.duration = 100;
NSData *data = [NSData dataWithBytes:&info length:sizeof(info)];
[data getBytes:&myContext length:sizeof(myContext)];
to pack the struct to a NSData and then write the bytes to myContext.
Then, I try to recreate the information using the 64bit integer as follows:
NSData *newData = [NSData dataWithBytes:&myContext length:sizeof(myContext)];
GameInfo *result = (GameInfo*) [newData bytes];
however, when I log out the values, I see that I'm only able to capture the first two values (gameMode and destroyed). If I add more variables to the struct, I still only capture the first 2 variables.
What am I doing wrong? Is there a smarter way to do this?

Answer 1

You are trying to pack 96 bits of data (3 unsigned ints on iOS / ARM) into a 64 bit container. So you see the first two 32-bit values and not the third.
Maybe you could try using shorts or chars, depending on the range of values your struct will hold, and try to get the stuct's size down to < 64 bits. I think 3 char on ARM will get aligned out to 48 bits (might be wrong about that if so please let me know!). So maybe use short anyway.
EDIT: example of possible way to change your struct, assuming you'll only store 16-bit values in each field:
typedef struct{
uint16_t gameMode;
uint16_t destroyed;
uint16_t duration;
} GameInfo;