TweetFollow Us on Twitter

Debugging Retain Count Bugs

Volume Number: 21 (2005)
Issue Number: 4
Column Tag: Programming

Tips From Big Nerd Ranch

by Aaron Hillegass, Chris Campbell, and Marquis Logan

Debugging Retain Count Bugs

At the Big Nerd Ranch, we teach the world's greatest Cocoa programming course (and the greatest Python, PHP, Perl, and PostgreSQL classes). One of the things that makes the course great is the alumni mailing list. We thought it would be a great service to the larger developer community if we shared some of the tips that have appeared on the list. So each month, we are going to cull through the discussion and publish a few pieces of useful wisdom. This month, we are talking about how to debug errors in the use of retain and release.

Hunting down bugs in your use of retain and release can be very difficult. Before delving into all the ways to find them, let's quickly review the lifetime of an object on the heap. The object is allocated (through the +allocWithZone: method which calls the malloc_zone_calloc() function), it is used, and then it is deallocated (through the -dealloc method which calls the malloc_zone_free() function).

Complicating matters is the retain count. A newly allocated object has a retain count of 1. When an object is retained, the retain count is incremented. When it is released, the retain count is decremented. When the retain count goes to zero, the object is deallocated.

This is complicated even further by the autorelease pool. When an object is sent the message autorelease, it adds itself to the current autorelease pool. When the autorelease pool is deallocated, it send the message release to every object in the pool. In a Cocoa application, the autorelease pool is deallocated after each event is handled.

Use release when possible

Here are two similar erroneous methods:

- (void)bad
{
    NSArray *array = [[NSArray alloc] init];
    [array release];
    [array release];
}

- (void)worse
{
    NSArray *array = [[NSArray alloc] init];
    [array release];
    [array autorelease];
}

The first method is run inside a debugger, it will stop on the erroneous line with a BAD_ACCESS error. The second method will cause a BAD_ACCESS only when the autorelease pool is deallocated. In a large program, it may be difficult to back track to where you autoreleased that object. Using release, besides being more efficient, will make your code easier to debug.

Note that this does not mean that you should break conventions: when creating an object to be vended out, it should be autoreleased:

- (NSNumber *)currentPrice
{
    NSNumber *p = [[NSNumber alloc] initWithInt:(whatTheMarketWillBear - 1)];
    [p autorelease];
    return p;
}

Set Pointers to nil after Objects are Released

To make it impossible to send messages to released objects, set the pointer to nil after the object is released:

- (void)fireThem
{
   [employees release];
   employees = nil;
}

Using Zombies

This code (which uses an object after it has been deallocated) often runs without crashing or throwing exceptions:

- (IBAction)stupidAction:(id)sender
{
    NSMutableString *string = [[nameField stringValue] mutableCopy];
    [string release];
    NSMutableString *anotherString = [NSMutableString stringWithFormat:@"I'm new!"];
    NSLog(@"string = %@", string);
}

The new string object is sometimes created in the same place in memory that the old one had occupied. So, when you run this you sometimes see:

string = I'm new!

On the other hand, sometimes the new string isn't created in the same location and the application crashes with a BAD_ACCESS. As you can imagine, hunting down a bug may or may not rear its ugly head can be maddening.

To debug this code, instead of freeing the memory for reuse, turn the objects into zombies. When you try to use a zombie, Cocoa will throw an exception. Thus, you can consistently reproduce this bug. The trick, then, is to tell Foundation and CoreFoundation, that you want to zombify your objects instead of freeing them.

First, you need to use the debug version of the CoreFoundation framework. To do this, select your executable in Xcode, and in the inspector under the General tab, choose "Use the debug suffix when loading frameworks."

Then, under the Arguments tab, set the environment variable NSZombieEnabled to YES and CFZombieLevel to 5:

Now when you send a message to a released object you will get a nice exception like this:

*** Selector 'respondsToSelector:' sent to dealloced instance 0x328840 of class NSMutableString.

(Remember to disable zombies once the bug is found -- you want the objects to be freed, not zombified in a deployed app.)

Putting a breakpoint on exceptions

Now that you have an exception that is thrown consistently, you will need to create a breakpoint in the debugger where the exception gets raised. Before dealing with the debugger, let's review how exceptions get thrown.

The Cocoa libraries check certain conditions, and if the conditions are not met, they raise exceptions. For example, if you ask for the second item in an array that has only one item, an exception will be thrown. Throwing an exception looks like this:

if (x == 13) {
    NSException *badness = [NSException exceptionWithName:@"BadLuckException"
                                                    reason:@"13 is unlucky"
                                                    userInfo:nil];
    [badness raise];
}

Thus, to stop the debugger as soon as an exception is raised, you can add the breakpoint for -[NSException raise] using the breakpoint panel.

Gradually, Cocoa programmers are moving to the new @throw/@catch/@finally form of exceptions stolen from C++ and Java. In this form, you will throw exceptions like this:

    NSException *badness = [NSException exceptionWithName:@"BadLuckException"
                                                   reason:@"13 is unlucky"
                                                   userInfo:nil];
    
    @throw badness;

(To use new-style exceptions, you must pass the -fobjc-exceptions flag to the compiler.)

To detect when an exception is being thrown from within the debugger, you will want to create a breakpoint on objc_exception_throw().

You will want these breakpoints every time you run the debugger. To automatically add these breakpoints anytime you use gdb, create a .gdbinit file in your home directory and include these lines:

fb -[NSException raise]
fb objc_exception_throw()

Those are the four tips for this week:

  • release instead of autorelease when possible
  • After releasing an object, set pointers to it to nil
  • Use zombies to hunt down retain/release problems
  • Add breakpoints to stop on exceptions

We hope they are useful to you, and we will be back next month with a few more.


Aaron Hillegass, Chris Campbell, and Marquis Logan

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Adobe Premiere Pro CC 2015 9.0.1 - Digit...
Premiere Pro CC 2015 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Premiere Pro customer). Premiere Pro CS6 is still available for... Read more
Adobe After Effects CC 2015 13.5.1 - Cre...
After Effects CC 2015 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous After Effects customer). After Effects CS6 is still available... Read more
Adobe Creative Cloud 2.2.0.129 - Access...
Adobe Creative Cloud costs $49.99/month (or less if you're a previous Creative Suite customer). Creative Suite 6 is still available for purchase (without a monthly plan) if you prefer. Introducing... Read more
Tower 2.2.3 - Version control with Git m...
Tower is a powerful Git client for OS X that makes using Git easy and more efficient. Users benefit from its elegant and comprehensive interface and a feature set that lets them enjoy the full power... Read more
Apple Java 2015-001 - For OS X 10.7, 10....
Apple Java for OS X 2015-001 installs the legacy Java 6 runtime for OS X 10.11 El Capitan, OS X 10.10 Yosemite, OS X 10.9 Mavericks, OS X 10.8 Mountain Lion, and OS X 10.7 Lion. This package is... Read more
Adobe Muse CC 2015 2015.0.1 - Design and...
Muse CC 2015 is available as part of Adobe Creative Cloud for as little as $14.99/month (or $9.99/month if you're a previous Muse customer). Muse CS6 is still available for purchase (without a... Read more
Adobe Illustrator CC 2015 19.1.0 - Profe...
Illustrator CC 2015 is available as part of Adobe Creative Cloud for as little as $19.99/month (or $9.99/month if you're a previous Illustrator customer). Illustrator CS6 is still available for... Read more
Corel Painter 14.1.0.1105 - Digital art...
Corel Painter helps you create astonishing art in a variety of media. Paint with vivid oil paints, fluid water colors, and earthy charcoals. Corel Painter flawlessly recreates the tones and textures... Read more
Pacifist 3.5.4 - Install individual file...
Pacifist opens up .pkg installer packages, .dmg disk images, .zip, .tar. tar.gz, .tar.bz2, .pax, and .xar archives and more, and lets you extract or install individual files out of them. This is... Read more
Dropbox 3.8.4 - Cloud backup and synchro...
Dropbox is an application that creates a special Finder folder that automatically syncs online and between your computers. It allows you to both backup files and keep them up-to-date between systems... Read more

Mazes of Karradash (Games)
Mazes of Karradash 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: The city of Karradash is under attack: the monsters of the Shadow Realms are emerging from the depths.No adventurer is... | Read more »
Battle Golf is the Newest Game from the...
Wrassling was a pretty weird - and equally great - little wressling game. Now the developers, Folmer Kelly and Colin Lane, have turned their attention to a different sport: golfing. This is gonna be weird. [Read more] | Read more »
Qbert Rebooted has the App Store Going...
The weird little orange... whatever... is back, mostly thanks to that movie which shall remain nameless (you know the one). But anyway it's been "rebooted" and now you can play the fancy-looking Qbert Rebooted on iOS devices. [Read more] | Read more »
Giant Monsters Run Amok in The Sandbox...
So The Sandbox has just hit version number 1.99987 (seriously), and it's added a lot more stuff. Just like every other update, really. [Read more] | Read more »
Fish Pond Park (Games)
Fish Pond Park 1.0.0 Device: iOS Universal Category: Games Price: $2.99, Version: 1.0.0 (iTunes) Description: Nurture an idyllic slice of tourist's heaven into the top nature spot of the nation, furnishing it with a variety of... | Read more »
Look after Baby Buddy on your Apple Watc...
Parigami Gold is the new premium version of the match three puzzler that includes Apple Watch support and all new content. You won't simply be sliding tiles around on your wrist, the Apple Watch companion app is an all new mini-game in itself. You'... | Read more »
Swallow all of your opponents as the big...
Eat all of the opposition and become the largest ball in Battle of Balls now available in the App Store and Google Play. Battle of Balls pits you against other opponents in real time and challenges you to eat more balls and grow larger than all of... | Read more »
PAC-MAN Championship Edition DX (Games)
PAC-MAN Championship Edition DX 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: It’s Your World. EAT IT! Get ready for more ghost chain gobbling and frantic action in PAC-MAN® CE-DX! The... | Read more »
incurve (Games)
incurve 1.0 Device: iOS Universal Category: Games Price: $.99, Version: 1.0 (iTunes) Description: Get ready for 2 different gravities Goal is to hit as many white dots on your way up.When you're touching the screen, the dots have a... | Read more »
Crossy Road has its Own Merch Store Now....
Do you like Crossy Road? I mean do you really like Crossy Road? Well then you're in luck! Hipster Whale has opened up a Crossy Road store, so you can show off your fandom via official T-shirts. [Read more] | Read more »

Price Scanner via MacPrices.net

Sale! 13″ 1.6GHz/256GB MacBook Air for $1099,...
B&H Photo has the 13″ 1.6GHz/256GB MacBook Air on sale for $1099 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model. Read more
iPad mini 4 To Be Upgraded To iPad Air 2 Spec...
There’s a certain inevitability about making Apple product predictions this time of year. Come September, we can pretty reliably count on the release of refreshed iPhones, along with the iOS 9... Read more
Apple restocks refurbished Mac minis for up t...
The Apple Store has restocked Apple Certified Refurbished 2014 Mac minis, with models available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz... Read more
13-inch 2.5GHz MacBook Pro on sale for $899,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $899.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $200 off MSRP. Price is... Read more
21-inch 2.9GHz iMac on sale for $1299, save $...
Best Buy has the 21″ 2.9GHz iMac on sale today for $1299.99 on their online store. Choose free shipping or free local store pickup (if available). Their price is $200 off MSRP, and it’s the lowest... Read more
Free Image Sizer 1.3 for iOS Offers Photo Edi...
Xi’An, China based G-Power has announced the release of Image Sizer 1.3 for the iPhone, iPad, and iPod touch, an important update to their free photo editing app. Image Sizer’s collection of easy to... Read more
Sale! 13″ 1.6GHz/128GB MacBook Air for $899,...
B&H Photo has the 13″ 1.6GHz/128GB MacBook Air on sale for $899 including free shipping plus NY tax only. Their price is $100 off MSRP, and it’s the lowest price available for this model. Read more
13-inch Retina MacBook Pros on sale for $100...
Best Buy has 13-inch Retina MacBook Pros on sale for $100 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Prices are for online orders only, in-store... Read more
Will BMW’s i3 Electric Vehicle Be The Automo...
The German-language business journal Manager Magazin’s Michael Freitag reports that Apple and the German performance/luxury automaker Bayerishe Motoren Werke (BMW) are back at far-reaching... Read more
Sale! $250 off 15-inch Retina MacBook Pro, $2...
B&H Photo has lowered their price for the 15″ 2.2GHz Retina MacBook Pro to $1749, or $250 off MSRP. Shipping is free, and B&H charges NY sales tax only. They have the 27″ 3.3GHz 5K iMac on... Read more

Jobs Board

*Apple* Customer Experience (ACE) Leader - A...
…management to deliver on business objectives Training partner store staff on Apple products, services, and merchandising guidelines Coaching partner store staff on Read more
Project Manager - *Apple* Pay Security - Ap...
**Job Summary** The Apple Pay Security team is seeking a highly organized, results-driven Project Manager to drive the development of Apple Pay Security. If you are Read more
*Apple* TV Product Design Internship (Spring...
…the mechanical design effort associated with creating world-class products with the Apple TV PD Group. Responsibilities will include working closely with manufacturing, Read more
*Apple* Watch SW Application Project Manager...
**Job Summary** The Apple Watch software team is looking for an Application Engineering Project Manager to work on new projects for Apple . The successful candidate Read more
*Apple* Retail - Multiple Positions (US) - A...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.