Question

Does anyone know what this error means?
*** attempt to pop an unknown autorelease pool

I am seeing this in my app that uses NSOperations in NSOperationQueue. Each operation has it’s own Autorelease pool. Since each operation is parsing xml and pushing information into Core Data, I periodically ‘drain’ my autorelease pool and re-create it. If I don’t drain the pool, I don’t see these errors.

UPDATE:
Here’s my code:

In the main of my NSOperation, I alloc the pool and assign it to an ‘assign’ property like this:

self.downloadAndParsePool = [[NSAutoreleasePool alloc] init];

at the end of my main I release it like this:

[self.downloadAndParsePool release];
self.downloadAndParsePool = nil;

If the operation does a lot of parsing and inserting into Core Data, it will call my drain method periodically:

- (void) drainAutoreleasePool
{
    // drain and re-create autorelease pool...this method is called periodically to keep memory down during parsing of large trees
    if (self.downloadAndParsePool)
    {
        [self.downloadAndParsePool drain];
        self.downloadAndParsePool = [[NSAutoreleasePool alloc] init];
    }
}

Answer 1

It seems that the issue is caused by the fact that the drain is called outside of the method that allocated the pool even if on the same thread. Here is what NSAutoreleasePool Class Reference says.

You should always drain an autorelease
pool in the same context (invocation
of a method or function, or body of a
loop) that it was created.

The message you see is triggered by your call to drain in drainAutoreleasePool:

[self.downloadAndParsePool drain];

NOTE: XMLPerformance sample app uses the same pattern of periodically draining the pool. It also produces this message since iOS4.0