EXC_BAD_ACCESS when adding NSOperation to NSOperationQueue

Question

I've been sitting on this error for hours now. I'm getting an EXC_BAD_ACCESS (code=2) on the line:

[self.downloadQueue addOperation:self.downloadOP];

I know it has to be related to memory conflicts, but I just can't find the problem. The class that manages the OperationQueues is a singleton, but i think thats not the problem.

Here's the shortened version of my .h file:

@interface GTMConnectionManager : NSObject{
}

@property (retain) GTMDownloadOperation *downloadOP;
@property (retain) NSOperationQueue *downloadQueue;
// it doesn't make a difference if I add 'nonatomic' to these properties

+ (GTMConnectionManager *)sharedConnectionManager;
-(void)downloadImageData:(NSMutableArray*)p_images andController:(UIViewController*)p_resultsController;

@end

And the important part of the .m file:

#import "GTMConnectionManager.h"
@implementation GTMConnectionManager
@synthesize downloadOP, downloadQueue;

+ (GTMConnectionManager *)sharedConnectionManager
{
    static GTMConnectionManager * instance = nil;

    static dispatch_once_t predicate;
    dispatch_once(&predicate, ^{
        instance = [[super allocWithZone:nil] init];
    });
    return instance;
}

-(void)downloadImageData:(NSMutableArray*)p_images andController:(GTMResultsListViewController*)p_resultsController{

    self.resultsController = p_resultsController;

    [self.downloadQueue setMaxConcurrentOperationCount:2];
    self.downloadQueue = [[[NSOperationQueue alloc]init]autorelease];
    // it doesn't make a difference if I do this with or without 'autorelease'

    for (int i = 0; i < [p_images count]; i++) {
        GTMGeoImage *tmpImg = [p_images objectAtIndex:i];
        self.downloadOP = [[[GTMDownloadOperation alloc]initWithImage:tmpImg]autorelease];
        [self.downloadQueue addOperation:self.downloadOP]; //Here's the error
    }
}

When I add a breakpoint just before the error-line, both self.downloadQueue and self.downloadOP are retained correctly (not nil).

The strange this is: in this very class I have a second NSOperationQueue with other NSOperations that are declared and treated in the same way as downloadQueue and downloadOP. And they work perfectly.

And yes, GTMDownloadOperation is a child-class of NSOperation and has a -(void)main method.

I don't know what to do now. If you don't have any idea about the reason of that error, how can I analyze the situation more accurately? (Product > Analyze doesn't complain about potential leak at that position).

Thanks for your help.

How To Debug Memory Leaks with XCode and Instruments Tutorial: http://www.raywenderlich.com/2696/how-to-debug-memory-leaks-with-xcode-and-instruments-tutorial — kaspartus, Sep 25 '12 at 09:54
Enable NSZombie to track EXC_BAD_ACCESS See: [Enable NSZombie in XCode][1] [1]: http://stackoverflow.com/questions/5386160/how-to-enable-nszombie-in-xcode — Abdullah Umer, Sep 25 '12 at 10:08

score 1 · Answer 1 · answered Sep 26 '12 at 06:11

1

Oh man... That took a while, but finally I know that the problem was in the for-loop.

The statement

self.downloadOP = [[[GTMDownloadOperation alloc]initWithImage:tmpImg]autorelease];

was accessing the variable downloadOP again and again in every iteration. Using the same NSOperation seemed to crash its retainCount.

I changed it to

GTMDownloadOperation *downloadOP = [[GTMDownloadOperation alloc]initWithImage:tmpImg];

and it works without error. Silly me.

answered Sep 26 '12 at 06:11

Chris

489
5
15

`retainCount` is nonsense; don't call it. Your code is still buggy as described by @meronix. You should understand why your code seems to be working now. My guess is that it isn't working correctly, just working by coincidence. – bbum Sep 26 '12 at 16:56

score 1 · Answer 2 · answered Jan 12 '16 at 09:17

1

you're not calling [super init] inside your constructor?

Assuming you're subclassing NSOperation (or NSObject etc...), you probably should!

answered Jan 12 '16 at 09:17

Siddu Halake

353
1
3
10

score 0 · Answer 3 · answered Sep 25 '12 at 09:50

why do you realloc and init your queue? couldn't it be just one for your singleton class?

and...

[self.downloadQueue setMaxConcurrentOperationCount:2];
self.downloadQueue = [[[NSOperationQueue alloc]init]autorelease];

the first line is executed on an old queue, then you create a new one that have no limits

when do you call the second line (alloc a new queue) you release the old one (if any)

try this:

if (!downloadQueue){
    self.downloadQueue = [[[NSOperationQueue alloc]init]autorelease];
}
[self.downloadQueue setMaxConcurrentOperationCount:2];

and remember to add:

self.downloadQueue = nil;

in your dealloc method (even if it's a singleton, and it won't be called while your app is running)

unfortunately this didn't help. Changing the code like you said didn't change anything about the error. — Chris, Sep 25 '12 at 10:24

score 0 · Answer 4 · answered Sep 25 '12 at 11:48

I believe the problem is in the implementation of your operation. I suggest two courses: try creating some simple block based operations that just log hello world and add them to the queue. They will most likely wok, letting you know the queue is functional. Then start adding log messages to you subclass to see which methods get called and finish properly.

This should lead you to the problem.

EXC_BAD_ACCESS when adding NSOperation to NSOperationQueue

4 Answers4