Question

I was just writing some exploratory code to solidify my understanding of Objective-C and I came across this example that I don’t quite get. I define this method and run the code:

- (NSString *)stringMethod
{
    NSString *stringPointer = [[NSString alloc] initWithFormat:@"string inside stringPointer"];
    [stringPointer release];
    [stringPointer release];
    NSLog(@"retain count of stringPointer is %i", [stringPointer retainCount]);
    return stringPointer;
}

After running the code and calling this method, I notice a few things:

1. Normally, if I try to access something that’s supposedly dealloced after hitting zero retain count, I get a EXC_BAD_ACCESS error. Here, I get a malloc “double free” error instead. Why is that?

2. No matter how many lines of “[stringPointer release]” I add to the code, NSLog reports a retain count of 1. When I add more releases I just get more “double free” errors. Why aren’t the release statements working as expected?

3. Although I’ve over-released stringPointer and I’ve received a bunch of “double free” errors, the return value still works as if nothing happened (I have another NSLog in the main code that reports the return value). The program continues to run normally. Again, can someone explain why this happens?

These examples are fairly trivial, but I’m trying to get a full grasp of what’s going on. Thanks!

Answer 1

You’re getting a double free error because you are releasing twice and causing two dealloc messages. =P

Keep in mind that just because you release doesn’t doesn’t mean the data at its memory address is immediately destroyed. It’s just being marked as unused so the kernel knows that, at some point in the future, it is free to be used for another piece of data. Until that point (which is totally nondeterministic in your app space), the data will remain there.

So again: releasing (and dealloc’ing) doesn’t necessitate immediate data destruction on the byte level. It’s just a marker for the kernel.