Question

I am debugging a weird memory management error and I can’t figure it out. I noticed that some of my objects are staying in memory longer than expected. I checked all my memory management and finally got to the very improbable conclusion that some of my autorelease operations don’t result in a release. Under what circumstances is that possible? I created a small testing Canary class that logs a message in dealloc and have the following testing code in place:

NSLog(@"On the main thread: %i.", [NSThread isMainThread]);
[[[Canary alloc] init] autorelease];

According to the code we’re really on the main thread, but the dealloc in Canary does not get called until much later. The delay is not deterministic and can easily take seconds or more. How is that possible? The application runs on a Mac, the garbage collection is turned off (Objective-C Garbage Collection is set to Unsupported on the target.) I am mostly used to iOS, is memory management on OS X different in some important way?

Answer 1

There’s a blog post by Mike Ash called More Fun With Autorelease that explains the source of this problem. Quote:

As everybody knows, every time you go
through the event loop, Cocoa blows
away the old pool and makes a new one
for you, so that all of your
autoreleased objects go away and your
new ones go into a fresh pool. That
way you never build up more objects
than get produced during a single
event loop cycle. The key word is “event loop”. In
Apple’s infinite wisdom, things that
aren’t real actual NSEvents don’t
trigger the pool.

I’m currently working on an app that
spends a lot of time in the background
doing dark, unspeakable things with
NSStreams on the main thread. I
encountered a bug where one of my
objects can get destroyed in the
middle of handling a stream event,
which left it open to getting other
stream events after it was
deallocated. (…)

The obvious fix was to simply do
[[self retain] autorelease] before
making the problem call. And fix it it
did, except instead of my dealloc
happening in the middle of my event
handler, it never happened at all.

Until I clicked on my app’s dock icon.

At least the solution was easy. Post
an NSApplicationDefined event in the
stream event handler, and autoreleased
objects get destroyed on schedule.

I swear I have to read the blog from cover to cover, it’s a good time investment.