ARC is enabled but having Memory Leak (Objective C) - objective-c

Question

As you can see, the code below isnt doing much (all commented out) more than enumerating over a set of files, however, my memory usage is growing to over 2 GB after 40 seconds of running the function below which is launched by pressing a button on the UI.
I can run the UI for hours, and before pressing the button, the memory usage does not exceed 8MB.
Given that ARC is turned on, what is holding on to the memory?
removed original code as the edit below made no differance.
EDIT:
Attempted #autoreleasepool{ dispatch_asyny ... } and permutations of that around the while and inside the while loop which had no effect.
Here is the code with autorelasepool added and cleaned up
-(void) search{
self.dict = [[NSMutableDictionary alloc] init];
NSFileHandle *fileHandle = [NSFileHandle fileHandleForWritingAtPath:#"/tmp/SeaWall.log"];
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
NSString *bundleRoot = #"/";
NSFileManager *manager = [NSFileManager defaultManager];
NSDirectoryEnumerator *direnum = [manager enumeratorAtPath:bundleRoot];
NSString *filename;
while ((filename = [NSString stringWithFormat:#"/%#", [direnum nextObject]] ) && !self.exit) {
#autoreleasepool {
NSString *ext = filename.pathExtension;
if ([ext hasSuffix:#"so"] || [ext hasSuffix:#"dylib"] ) {
if (filename == nil || [NSURL URLWithString:filename] == nil) {
continue;
}
NSData *nsData = [NSData dataWithContentsOfFile:filename];
if (nsData != nil){
NSString *str = [nsData MD5];
nsData = nil;
[self writeToLogFile:[NSString stringWithFormat:#"%# - %#", [filename lastPathComponent], str]];
}
}
ext = nil;
} // end autoreleasepool
}
[fileHandle closeFile];
[self ControlButtonAction:nil];
});
}

Answer 1

The memory is not exactly leaked: it is very much ready to be released, but it never has a chance to be.
ARC builds upon the manual memory management rules of Objective-C. The base rule is that "the object/function that calls init owns the new instance", and the owner must release the object when it no longer needs it.
This is a problem for convenience methods that create objects, like [NSData dataWithContentsOfFile:]. The rule means that the NSData class owns the instance, because it called init on it. Once the value will be returned, the class will no longer need the object, and it would need to release it. However, if this happens before the callee gets a chance to retain the instance, it will be gone before anything had a chance to happen.
To solve this problem, Cocoa introduces the autorelease method. This method transfers the ownership of the object to the last autorelease pool that was set up. Autorelease pools are "drained" when you exit their scope.
Cocoa/AppKit/UIKit automatically set up autorelease pools around event handlers, so you generally do not need to worry about that. However, if you have a long-running method, this becomes an issue.
You can declare an autorelease pool using the #autoreleasepool statement:
#autoreleasepool
{
// code here
}
At the closing bracket, the objects collected by the autorelease pool are released (and possibly deallocated, if no one else has a reference to them).
So you would need to wrap the body of your loop in this statement.
Here's an example. This code "leaks" about 10 megabytes every second on my computer, because the execution never leaves the #autoreleasepool scope:
int main(int argc, const char * argv[])
{
#autoreleasepool
{
while (true)
{
NSString* path = [NSString stringWithFormat:#"%s", argv[0]];
[NSData dataWithContentsOfFile:path];
}
}
}
On the other hand, with this, the memory usage stays stable, because execution leaves the #autoreleasepool scope at the end of every loop iteration:
int main(int argc, const char * argv[])
{
while (true)
{
#autoreleasepool
{
NSString* path = [NSString stringWithFormat:#"%s", argv[0]];
[NSData dataWithContentsOfFile:path];
}
}
}
Creating objects in the loop condition is awkward for long loops because these are not picked up by the inner #autoreleasepool. You will need to get these inside the #autoreleasepool scope as well.
Returning
Whenever we return an object (maybe to Swift), we need to register into nearest #autoreleasepool block (by calling autorelease method to prevent memory-leak, according to ownership-rules), but nowadays ARC does that automatically for us;
Whenever ARC disabled; after using alloc and/or init, call autorelease manually, like:
- (NSString *)fullName {
NSString *string = [[[NSString alloc] initWithFormat:#"%# %#",
self.firstName, self.lastName] autorelease];
return string;
}

Answer 2

Memory needs to be released by an autorelease pool.
Otherwise it will be locked up as you are experiencing and it will leak.
In your loop put:
#autoreleasepool { /* BODY */ }