stringWithContentsOfURL leaking memory

Question

Would it shed more light if I told that fetchHTML was being called in a seperate thread? I am also seeing several messages in the debug console such as:

_NSAutoreleaseNoPool(): Object 0xd92860 of class NSCFDictionary autoreleased with no pool in place - just leaking

_NSAutoreleaseNoPool(): Object 0xd92800 of class NSCFString autoreleased with no pool in place - just leaking

I am new to iPhone app development, Objective-C but not new to programming or C/C++. I am using the leaks performance tool and it shows many leaks. This is a 10.5 kb leak and it occurs on the line:

NSString * xml = [NSString stringWithContentsOfURL:urlobj];

The stack trace on this below is:

stringWithContentsOfURL
initWithContentsOfURL
initWithDataOfEncoding
...

Does anyone have an idea why this must be happening. I am under the impression that I get an autorelease object here and I can return this to the caller without calling retain. I am not using the xml object to store in an instance variable, just for processing.

Here is the function code:

- (NSString *) fetchHTML: (NSString* ) url{
    @try
    {
        NSURL* urlobj = [NSURL URLWithString:url];
        NSString * xml = [NSString stringWithContentsOfURL:urlobj];
        return xml;
    }
    @catch( NSException *ex){
        NSLog(@"Error fetchingHTML");
        return nil;
    }
    return nil;
}

Answer 1

Yup; that shouldn't be leaking.

It might be a false positive in that the URL subsystem is caching the contents of the URL and doing so in a way where the pointer is no longer visible to leaks analysis.

If you can, retry the test on Snow Leopard. Leaks detection on Snow Leopard is significantly faster and more accurate.

Answer 2

I completely agree with you that this should not cause a leak. I've been coding in Cocoa/Objective-C for 2 years now, and that looks like it should work.

That being said, I notice that Apple's documentation indicates that the stringWithContentsOfURL: method is being deprecated. Perhaps it would work as follows:

NSString * xml = [[NSString alloc]
                  initWithContentsOfURL:urlobj
                               encoding:NSASCIIStringEncoding
                                  error:nil];
return [xml autorelease];

Answer 3

As the error message says, there's no autorelease pool for the string to go into, and that creates a leak. NSAutoreleasePools exist on a per-thread basis. Cocoa creates one in the main event loop of the main thread, but that's the only one it creates for you. If you're somewhere other than the main thread and you're going to be dealing with autoreleased objects, you need to create an autorelease pool for that thread as well.

You can check out the NSAutoreleasePool docs for more information on how autorelease pool stacks work.