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

Things 3.1.3 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
BetterTouchTool 2.292 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Things 3.1.3 - Elegant personal task man...
Things is a task management solution that helps to organize your tasks in an elegant and intuitive way. Things combines powerful features with simplicity through the use of tags and its intelligent... Read more
BetterTouchTool 2.292 - Customize Multi-...
BetterTouchTool adds many new, fully customizable gestures to the Magic Mouse, Multi-Touch MacBook trackpad, and Magic Trackpad. These gestures are customizable: Magic Mouse: Pinch in / out (zoom... Read more
Bookends 12.8.3 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Mellel 3.5.5 - The word processor for sc...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
Mellel 3.5.5 - The word processor for sc...
Mellel is the leading word processor for OS X and has been widely considered the industry standard since its inception. Mellel focuses on writers and scholars for technical writing and multilingual... Read more
Bookends 12.8.3 - Reference management a...
Bookends is a full-featured bibliography/reference and information-management system for students and professionals. Bookends uses the cloud to sync reference libraries on all the Macs you use.... Read more
Carbon Copy Cloner 4.1.18 - Easy-to-use...
Carbon Copy Cloner backups are better than ordinary backups. Suppose the unthinkable happens while you're under deadline to finish a project: your Mac is unresponsive and all you hear is an ominous,... Read more
Hopper Disassembler 4.2.14- - Binary dis...
Hopper Disassembler is a binary disassembler, decompiler, and debugger for 32- and 64-bit executables. It will let you disassemble any binary you want, and provide you all the information about its... Read more

Little Red Lie (Games)
Little Red Lie 1.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0 (iTunes) Description: ARE YOU MORE AFRAID OF POVERTY THAN DEATH? Little Red Lie is a narrative-focused, interactive fiction experience that reduces... | Read more »
You can now apply to be Clash of Clans...
Earlier this month, word got out that the Builder, the trusty handiman who tirelessly built every single building inevery singleClash of Clansbase had called it quits. Sick of seeing his work destroyed endless, the Builder has set out for our world... | Read more »
Meshi Quest beginner's guide - how...
Meshi Quest is Square Enix's newest free-to-play release, and it's a real charmer. You start off as the head of a sushi restaurant, upgrading your food and equipment as you serve visitors heaping helpings of your delicious meals. As you progress,... | Read more »
BUST-A-MOVE JOURNEY (Games)
BUST-A-MOVE JOURNEY 1.0.0 Device: iOS Universal Category: Games Price: $4.99, Version: 1.0.0 (iTunes) Description: BUST-A-MOVE Features:- Shoot bubbles and match 3 or more bubbles of the same color to make them pop!- Complete your... | Read more »
The best card combos in Clash Royale
Clash Royale is all about building a deck of units that synergise well. To help you get off to a flying start, we've put together a list of unit combinations that are incredibly effective. Looking for some choice 2v2 combos? Check out our guide. [... | Read more »
The best 2v2 card combos in Clash Royale
2v2 is making it's grand return toClash Royalequite soon. 2v2 has quickly become one of the game's most popular gameplay modes, though they still have yet to make it a permanent fixture in the game. 2v2 is exciting and adds some new flavor to... | Read more »
The best games we played this week - Aug...
Another busy week has come to a close. We played a lot of excellent games this week and now it's time to look back and reflect on some our favorites. Here are our picks for the week of August 18. [Read more] | Read more »
War Wings beginner's guide - how to...
War Wings is the newest project from well-established game maker Miniclip. It's a World War II aerial dogfighting game with loads of different airplane models to unlock and battle. The game offers plenty of single player and multiplayer action. We... | Read more »
How to win every 2v2 battle in Clash Roy...
2v2 is coming back to Clash Royale in a big way. Although it's only been available for temporary periods of time, 2v2 has seen a hugely positive fan response, with players clamoring for more team-based gameplay. Soon we'll get yet another taste of... | Read more »
Roll to Win with Game of Dice’s new upda...
Joycity’s hit Game of Dice gets a big new update this week, introducing new maps, mechanics, and even costumes. The update sets players loose on an exciting new map, The Cursed Tower, that allows folks to use special Runes mid-match. If you feel... | Read more »

Price Scanner via MacPrices.net

Free Instant Translator 2.0 App For iOS Relea...
Mobile application development company, Neoappz has announced the release and immediate availability of Instant Translator 2.0 for iOS devices. Instant Translator is a user-friendly application which... Read more
2017 15-inch MacBook Pros on sale for $200 of...
Amazon has 2017 15″ MacBook Pros on sale for $200 off MSRP. Shipping is free: – 15″ 2.8GHz MacBook Pro Space Gray: $2199.99, $200 off MSRP – 15″ 2.8GHz MacBook Pro Silver: $2296, $103 off MSRP – 15″... Read more
Apple’s 2017 Back to School Promotion: Free B...
Purchase a new Mac using Apple’s Education discount, and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free. As part... Read more
Clearance 2016 12-inch Retina MacBooks, Apple...
Apple has Certified Refurbished 2016 12″ Retina MacBooks available starting at $1019. Apple will include a standard one-year warranty with each MacBook, and shipping is free. The following... Read more
15-inch 2.2GHz Retina MacBook Pro, Apple refu...
Apple has Certified Refurbished 2015 15″ 2.2GHz Retina MacBook Pros available for $1699. That’s $300 off MSRP, and it’s the lowest price available for a 15″ MacBook Pro. An Apple one-year warranty is... Read more
Apple refurbished Mac minis available startin...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: – 1.4GHz Mac mini: $419 $80 off MSRP – 2.6GHz Mac... Read more
Apple refurbished iPad Pros available startin...
Apple has Certified Refurbished 2016 12″ WiFi iPad Pros available starting at $589. An Apple one-year warranty is included with each model, and shipping is free: – 32GB 12″ iPad Pro WiFi: $589... Read more
Weekend sale: 13-inch MacBook Pros for up to...
Amazon has new 2017 13″ MacBook Pros on sale today for up to $200 off MSRP, each including free shipping: – 13″ 3.1GHz/256GB Space Gray MacBook Pro (MPXV2LL/A): $1599.99 $200 off MSRP – 13″ 3.1GHz/... Read more
Back To School With The Edge Desk All-in-one...
Back to school is just around the corner, and the ergonomically correct Edge Desk all-in-one portable kneeling desk is ideal for students living in dorms and small apartments, Edge Desk features:... Read more
Norton Core Secure Wi-Fi Router Now Available...
First introduced at the 2017 Consumer Electronics Show (CES), Norton Core, a secure, high-performance Wi-Fi router, fundamentally changed the concept of Wi-Fi routers by making security the primary... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
Development Operations and Site Reliability E...
Development Operations and Site Reliability Engineer, Apple Payment Gateway Job Number: 57572631 Santa Clara Valley, California, United States Posted: Jul. 27, 2017 Read more
Frameworks Engineering Manager, *Apple* Wat...
Frameworks Engineering Manager, Apple Watch Job Number: 41632321 Santa Clara Valley, California, United States Posted: Jun. 15, 2017 Weekly Hours: 40.00 Job Summary Read more
*Apple* Solutions Consultant - Apple Inc. (U...
…about helping others on a team while also delighting customers? As an Apple Solutions Consultant (ASC), you will discover customers needs and help connect them Read more
*Apple* Solutions Consultant - Apple Inc. (U...
Job Summary As an Apple Solutions Consultant, you'll be the link between our future customers and our products. You'll showcase your entrepreneurial spirit as you Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.