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Split NSData objects into other NSData objects with a given size

Answer 1

+2 A:

The following piece of code does the fragmentation without copying the data:

NSData* myBlob;
NSUInteger length = [myBlob length];
NSUInteger chunkSize = 100 * 1024;
NSUInteger offset = 0;
do {
    NSUInteger thisChunkSize = length - offset > chunkSize ? chunkSize : length - offset;
    NSData* chunk = [NSData dataWithBytesNoCopy:(void*)[myBlob bytes] + offset
                                         length:thisChunkSize
                                   freeWhenDone:NO];
    offset += thisChunkSize;
    // do something with chunk
} while (offset < length);

Sidenote: I should add that the chunk objects cannot safely be used after myBlob has been released (or otherwise modified). chunk fragments point into memory owned by myBlob, so don't retain them unless you retain myBlob.

Nikolai Ruhe 2010-05-24 17:58:29

I'm not completely getting that. `NSData *chunk = [NSData dataWithBytesNoCopy:[myBlob bytes] + offset length:thisChunkSize freeWhenDone:NO];` is throwing a warning of course. I don't think you can say `[myBlob bytes] + offset`. Offset has nothing to do with bytes. I don't seem to understand your logic here.

Cedric Vandendriessche 2010-05-24 18:12:35

I edited the code to remove the compiler warning. The rest of the code seems right to me, though I wrote it without testing. `offset` is the amount of bytes already processed. That's why every new chunk is initialized at the beginning of the data array plus the `offset`.

Nikolai Ruhe 2010-05-24 19:17:23

You're saying that the `chunk` objects point into memory owned by `myBlob`. Does this mean when I transfer chunks over bluetooth they'll be corrupted on the other side? Or do you mean that I can send them but they won't be available anymore on the sender's side after releasing `myBlob`?

Cedric Vandendriessche 2010-05-29 11:16:58

It's all about your process. In this process, chunk objects would point to abandoned memory if `myBlob` would be released before the chunk object. On the receiver side, the address space and memory management of your process has no relevance.

Nikolai Ruhe 2010-05-29 12:01:29

Thanks for your help, all is working fine. If I use `NSData *chunk = [NSData dataWithBytes:(void*)[myBlob bytes] + offset length:thisChunkSize];` I can release the blob. May I ask why you chose the `-dataWithBytesNoCopy:`?

Cedric Vandendriessche 2010-05-29 15:36:46

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Split NSData objects into other NSData objects with a given size

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