What might cause half of an NSData object to stay in memory after releasing it?

Question

I'm loading a series of images from a server into NSData objects like so:

for (int i = 0; i < 36; i++)
{
 NSURL *url = [NSURL URLWithString:@"http://12.34.56.78/image.jpg"];
 NSData *data = [NSData dataWithContentsOfURL:url];
    // Further processing here
}

The problem is that half of each data object is being kept in memory. This does not show up as a leak in instruments. I know it's the NSData object because I have removed everything having to do with images and really only have the two lines before the comment now. The same behavior occurs. I've tried alloc initing and releasing explicitly with the same result.

The thing that makes this really hard to figure out is that I created a second project to try to recreate this behavior and I can't get it to do so. In the other project, this code acts as expected. So I'm asking, what might cause such behavior? I feel like I'm overlooking something extremely obvious.

Answer 1

From the two lines you have written, that data object should never leak because you are not retaining it, when you go out of scope that d ata object should autorelease...So cant really tell from the two lines you have posted..

Answer 2

I have encountered something similar where I had an array in my AppDelegate and I was grabbing a reference to a single row then (mistakenly) releasing my handle on the object. The result was that after 3 subsequent calls, the object in the row in question had nil values in all properties but itself was not nil. Took me about a week to figure that one out. To this day I still have no idea why it took 3 calls to release before I noticed a problem. I'm sure you can imagine my frustration when a week later I realized that one line of code was the source of 20 or so wasted hours. ;)

Answer 3

If what you're seeing is steadily growing memory, use Instruments' Object Allocations probe, and look for what is actually holding the memory. There are many ways to waste memory in ways that are not a "leak." The fact that the size is half the NSData size suggests that you're looking in the wrong place. It is unlikely that you are freeing half an object.