TweetFollow Us on Twitter

Developer to Developer: Thanks for the Memory-Part 2

Volume Number: 26
Issue Number: 11
Column Tag: Developer to Developer

Developer to Developer: Thanks for the Memory-Part 2

A look at how memory is managed in Objective-C

by Boisy G. Pitre

Introduction

If you followed last month's Developer to Developer column, you'll recall that we went through an introduction to memory management in Objective-C and Cocoa applications. That article was well suited to those who were new to Mac and iOS development, and touched on a number of concepts that are unique to Apple's development environment. For comparison, we also looked at how memory management is done in C and C++. Also, we touched on how Java programmers are greatly insulated from any memory considerations due to garbage collection.

This month we'll build upon the previous article and wade a little deeper into the waters of Objective-C memory management. We will start out by looking at a special subclass of NSObject that we can use to see just how our objects behave when they are created, retained, released and deallocated. We'll also take a look at the NSAutoreleasePool class, which gives us a convenient way to defer the release of an object and the return of its memory to the system. Finally, we will take Instruments, Apple's robust developer tool, for a spin.

Seeing Is believing

As we discussed last month, memory management in Objective-C consists of requesting an ownership interest in an object by retaining it, and relinquishing that ownership interest by releasing it. These operations can occur numerous times within an application for the same object. Balancing them is key to insuring that the memory that an object occupies is not left to leak while the application runs. Conversely, we must guard against the memory being returned to the system too soon, which could result in a crash, depending upon how that object is accessed later.

How convenient would it be to actually visualize this process happening in real-time? Well, it can be done, and quite readily, by subclassing NSObject and creating a new base class which overrides the retain and release methods, among others. And that's exactly what we will do. I highly recommend that you follow along by downloading the code for this month's article from the MacTech FTP source archive at ftp://ftp.mactech.com and launching the project file in Xcode. The application is made up of simple, contrived examples that are useful in illustrating the mechanics of object allocation and deallocation.

When determining what methods to override, we can consult two resources: the NSObject Class Reference (available in the Xcode's Developer Documentation, accessible from the Xcode Help menu) or the NSObject.h header file itself. Let's take the header file route and take a peek at NSObject.h directly. To do this easily from within Xcode, go to the File > Open Quicky... menu option, then type NSObject.h. It should appear in the drop down box; select it and it will be displayed in an Xcode text editor window.


Figure 1 - Opening the NSObject.h header file

The header file is composed of several sections: basic protocols, the base class, discardable content and object allocation/deallocation. For this article, we're going to focus on methods in the basic protocols and base class sections of the header file (starting at lines 11 and 63 respectively of the header file in the 10.6 SDK). Of the methods declared in the basic protocols section, let's override the following:

- (id)retain;
- (oneway void)release;
- (id)autorelease;

Similarly, let's override the following methods declared in the base class section:

- (id)init;
- (void)dealloc;

Lastly, for informational purposes, we'll override this method:

+ (id)allocWithZone:(NSZone *)zone;

There may be some questions as to the choice of methods to override. Remember, we are trying to capture the memory management operations in a way that allows us to visually confirm them as they happen. Most of the above methods are vectors for the retain count changing. By overriding these methods with methods in a subclass that (a) calls the same method in NSObject, and (b) logs the call itself and the retain count, we can see Objective-C memory management in action.

As mentioned earlier, overriding the methods requires subclassing NSObject. We'll create a new class just for the Developer to Developer column, DDObject, as a direct descendant of NSObject, so any class who inherits from DDObject will automatically benefit from the methods that we will embellish. Let's start out by looking at the code for the retain and release methods:

- (id)retain;
{
   id result = [super retain];
   NSLog(@"[%@ retain] (retainCount = %d)", [self className], [self retainCount]);
   return result;
}
- (oneway void)release;
{
   NSLog(@"[%@ release] (retainCount = %d)", [self className], [self retainCount] - 1) withLevel:TBLogLevelInfo];
   [super release];
}

Both methods mimic the return values of their original definitions in NSObject.h and use the NSLog() function to show the name of the class and the retain count. The retain method returns the retain count after the superclass' retain method is called, while the release method shows the retain count first. This ordering insures that we see the true retain count value at the appropriate place and time.

The init and dealloc methods are also points where observing the retain count can be instructive, so we'll extend these as well:

- (id)init;
{
   if (self = [super init])
   {
      NSLog(@"[%@ init] (retainCount = %d)", [self className], [self retainCount]);
   }
   
   return self;
}
- (void)dealloc;
{
   NSLog(@"[%@ dealloc]", [self className]);
   [super dealloc];
}

Even though we explicitly call retainCount in the init method, it will always print a retain count of 1. The dealloc method is called only when the retain count goes to 0; since a release call precedes this, we'll forego logging the retain count, and simply log that we are deallocating the object's memory here.

Finally, we'll override the allocWithZone: method, which will clue us in when an allocation is taking place (as we'll see shortly, the alloc method actually calls allocWithZone: with a zone of 0):

+ (id)allocWithZone:(NSZone *)zone;
{
   NSLog(@"[%@ allocWithZone:%d]", [self className], zone);
   return [super allocWithZone:zone];
}

Now that the pieces are in place, let's take a look at memexplore.m. This file contains the main() function, several test functions, and the interface and implementation for the MemObject class. This class extends the DDObject class which we just reviewed, so we would expect to see some interesting output when we run the test. Let's go ahead and do that now. First, build the memexplore target (Build > Build). Next, ensure that the Debugger Console is in focus so that the logging results can be seen (Run > Console). Finally, run the application (Run > Run). A menu appears where you can type the number of the test to perform. Let's run the allocRunRelease test by typing 1 then the return key.


Figure 2 – Output of the allocRunRelease test

The output of this test clearly shows the steps in which our MemObject comes to life, runs, then is finally released. As expected, the init method shows that the retain count is 1. The run method is then called, and finally the release method. Recall that this method decrements the retain count by 1; this is confirmed by the retain count falling to 0, and the dealloc method being implicitly called after that. Just to be convincing that the retain count can go higher, the allocRunReleaseMultiple test performs a retain after allocating and initializing the MemObject object, as well as an extra release. The net effect is the same: the object's dealloc method is called when the retain count falls to 0. Go ahead and run this test as well.


Figure 3 - Output of the allocRunReleaseMultiple test

One might conclude that the "hands-on" memory management aspect of Objective-C is a bit laborious. After all, we not only explicitly allocate and initialize an object, we must also take care to properly release it. Not releasing an object after we are done with it denies the use of that memory elsewhere; if an application doesn't release an object properly, it will stay around until the application quits, which could be a short time or a long time. Even though current systems contain gigabytes of memory and can accommodate a bit of sloppy memory management, it is considered bad programming practice to tolerate memory leaks. On mobile devices such as the iPhone and iPad where resources are limited, it is even more critical to wipe out these types of memory leaks.

Swimming In The NSAutoreleasePool

As we discussed, balancing retains with releases insures that we avoid memory leaks or crashes. It is a disciplined approach, and forces us to think about the lifetimes of our objects. However, Cocoa gives us a bit of a reprieve from the tedium of retain count management. We can make things a little easier for ourselves by conveniently deferring the return of an object's memory to the system using a special type of class provided by the Cocoa framework: NSAutoreleasePool.

An autorelease pool acts as a sort of dumping site for objects; upon receiving an object, the pool dutifully records a reference to the memory location of objects for later releasing. Any object can be relegated to the autorelease pool by having the autorelease message sent to it:

   SomeObject *s = [[[SomeObject alloc] init] autorelease];

The autorelease message allows us to pass complete responsibility of releasing the object to the NSAutoreleasePool. By doing this, we essentially wash our hands of further worry about the lifetime of the object and the memory that it is taking up. In the above code fragment, the object s receives the autorelease message after the alloc and init messages; subsequent to that, we can use the object as needed but should not send the release message to the object. That will be performed by the autorelease pool at a later time.

Exactly where autorelease pools are created depends upon the context in which you are writing your program. In Cocoa applications, an autorelease pool is created for you automatically, so you don't have to concern yourself with its creation. However, if you are using threads, you will need to create and manage your own autorelease pool for that thread. And in the case of a command line based program such as memexplore, it is necessary to create an autorelease pool as well.

Autorelease pools can be created many times, with the most recent pool being the one that receives any objects that receive the autorelease message. In essence, autorelease pools are stacked as they are created. When the topmost autorelease pool is destroyed, the next autorelease pool will receive autoreleased objects. Destroying an autorelease pool is similar to destroying any object: sending a release message to the pool will cause it to in turn send release messages to all objects that it holds, and finally the pool itself is deallocated. A slight twist on this is that since the release of Objective-C 2.0 and garbage collection, the drain message is the desired message to send to an autorelease pool instead. This message performs the same function as the release message, but does additional work in a garbage collected environment. For our code, we'll use the drain message when releasing our pools.

Can we peek into an autorelease pool? We certainly can, thanks to the NSDebug.h header file's NSAutoreleasePoolDebugging category. The showPool message, when sent to an autorelease pool object, will display the contents of all pools in the pool stack to the standard error path. We use this debugging method in several of our test programs to illustrate what the pool looks like just before it is drained. To illustrate this, run test 1 (allocRunRelease) then test 5 (allocRunReleaseWithPoolOverRetain). The final output will look like this:

==== top of stack ================
  0x100110b00 (NSCFString)
  0x100110ae0 (NSCFString)
  0x100110920 (MemObject)
  0x100110a70 (NSCFString)
  0x100110a50 (NSCFString)
  0x100110a30 (NSCFString)
==== top of pool, 6 objects ================
  0x1001109b0 (NSCFString)
  0x1001109e0 (NSCFString)
  0x100110990 (NSCFString)
  0x1001108b0 (NSCFString)
  0x100110070 (NSCFString)
==== top of pool, 5 objects ================

The pool appearing on the very top of the stack was created in the allocRunReleaseWithPoolOverRetain() function and has 6 objects, including a MemObject which we overretained and is just leaking. The next pool was created in the main() function has 5 objects. You may be wondering what is going on with all of the NSCFString objects in the autorelease pool. Those are the various string literals which appear in the NSLog() functions that have been executed during the program's run. As objects themselves, these strings require memory management, and they are automatically added to the autorelease pool when initialized.

Pool Hazards

As convenient as autorelease pools are, they must be used with care. The same problems that we discussed last month (overretaining/underreleasing or overreleasing/

underretaining) can still lead to memory leaks or crashes. A common mistake that many beginning programmers make is to send a release message to an object after it has been sent an autorelease message. The net effect of that transgression is that the object will end up being overreleased, and a crash is likely. The allocRunReleaseWithPoolOverRelease test illustrates this poignantly.

   NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init];
   NSLog(@"- allocRunReleaseWithPoolOverRelease -");
   MemObject *s1 = [[[MemObject alloc] init] autorelease];
   [s1 run];
   [s1 release];
   NSLog(@"----------------");
   [NSAutoreleasePool showPools];
   [pool drain];

Note that the autorelease message is sent to the s1 object upon creation, then a release message follows. It is at that point which the object's retain count goes to 0 and its dealloc method is called. Since its reference is also in the autorelease pool, the simple act of sending the showPools method is enough to trigger an access exception and crash the program.

Another interesting hazard is having no autorelease pool at all. Without an autorelease pool, objects sent an autorelease message (including NSCFString string literals that we saw above) have nowhere to go, and just leak all over the place. If you ever see a message like this coming from your application:

*** __NSAutoreleaseNoPool(): Object 0x100110770 of class NSCFString autoreleased with no pool in place - just leaking

then you know that somewhere, an autorelease pool is needed but doesn't exist. This most commonly happens when using threads and failing to create an autorelease pool. As a rule, your thread's entry point method should create an autorelease pool at the beginning, then drain the pool at the end.

Inspecting Memory Leaks With Instruments

Apple's Instruments is part of the Xcode development suite, and is a powerful tool for strengthening and bulletproofing your applications in a number of areas. When it comes to memory usage and management, it is particularly useful, and has handy two templates that are a must: Allocations and Leaks. The former template shows you exactly what objects and how many are being allocated by your application. The latter gives you insight into where your application may be leaking memory.

Typically, Instruments can be invoked from within Xcode's Run menu. Given that our application is a windowless application whose input and output appear on the console, we'll start Instruments from the Finder and then attach to the running process.

Before starting Instruments, go ahead and run the memexplore program from within Xcode. Now let's start Instruments by navigating to the /Developer/Applications folder in the Finder and double-click the Instruments icon. You will see a window with a drop-down sheet asking for the template to use. Select the Leaks template and click the Choose button.


Figure 4 - Selecting the Leaks template from Instruments

With the template chosen, you will see the main Instruments window with both the Allocations and Leaks templates shown. Since our application is running, we can attach to it from the Choose Target button and selecting the Attach to Process menu item, then navigate the list of running processes until we find memexplore. After memexplore has been selected, click the Record button in the toolbar. This starts the process of recording all object allocations as well as the leak detection procedure.


Figure 5 - Selecting the memexplore process from Instruments

With Instruments recording the memexplore application, switch to the Xcode console and select test 5 (allocRunReleasePoolWithPoolOverRetain). This test purposely performs an extra retain to the object so that it will leak. After the test is run, switch back to Instruments and click on the Leaks template header on the left side of the window. Within a a short time, the leaked object name should appear, along with the address and size. Clicking the third button of the view group in the toolbar will reveal the extended detail including the stack trace where the leak occurred. As we can see in the stack trace, the DDObject's allocWithZone: method is where the leak originated.


Figure 6 - Instruments showing the memory leak in memexplore

Summary

As we have seen, mismanaging an object's lifetime through its retain count can have ramifications for the health of your applications. Autorelease pools give us some convenience but even so, we must still be vigilant when balancing our retains and releases. Crashes are often caused by objects being released prematurely, and leaks are the result of retaining an object beyond its lifetime. It's times like these when Apple's Instruments can pinpoint the exact spot where the leak occurred, and we can take corrective action. For those of you who have started delving into Objective-C, these articles and the accompanying code should give you a basis for understanding memory management as well as a springboard to further experimentation.

Bibliography and References

Apple. Instruments User Guide. http://developer.apple.com/library/mac/documentation/DeveloperTools/Conceptual/InstrumentsUserGuide/InstrumentsUserGuide.pdf

CocoaDev. DebuggingAutorelease.

http://www.cocoadev.com/index.pl?DebuggingAutorelease


Boisy G. Pitre lives in Southwest Louisiana and is the lead developer at Tee-Boy where he also consults on Mac and iOS projects with a variety of clients. He holds a Master of Science in Computer Science from the University of Louisiana at Lafayette. Besides Mac programming, his hobbies and interests include retro-computing, ham radio, vending machine and arcade game restoration, and playing Cajun music. You can reach him at boisy@tee-boy.com.

 
AAPL
$103.39
Apple Inc.
+0.89
MSFT
$44.97
Microsoft Corpora
-0.46
GOOG
$574.33
Google Inc.
+2.73

MacTech Search:
Community Search:

Software Updates via MacUpdate

NetShade 6.0.2 - Browse privately using...
NetShade is an Internet security tool that conceals your IP address on the web. NetShade routes your Web connection through either a public anonymous proxy server, or one of NetShade's own dedicated... Read more
Mac DVDRipper Pro 5.0 - Copy, backup, an...
Mac DVDRipper Pro is the DVD backup solution that lets you protect your DVDs from scratches, save your batteries by reading your movies from your hard disk, manage your collection with just a few... Read more
pwSafe 3.1 - Secure password management...
pwSafe provides simple and secure password management across devices and computers. pwSafe uses iCloud to keep your password databases backed-up and synced between Macs and iOS devices. It is... Read more
StatsBar 1.8 - Monitor system processes...
StatsBar gives you a comprehensive and detailed analysis of the following areas of your Mac: CPU usage Memory usage Disk usage Network and bandwidth usage Battery power and health (MacBooks only)... Read more
Path Finder 6.5.5 - Powerful, award-winn...
Path Finder is a file browser that combines the familiar Finder interface with the powerful utilities and innovative features. Just a small selection of the Path Finder 6 feature set: Dual pane... Read more
QuarkXPress 10.2.1 - Desktop publishing...
With QuarkXPress, you can communicate in all the ways you need to -- and always look professional -- in print and digital media, all in a single tool. Features include: Easy to Use -- QuarkXPress is... Read more
Skype 6.19.0.450 - Voice-over-internet p...
Skype allows you to talk to friends, family and co-workers across the Internet without the inconvenience of long distance telephone charges. Using peer-to-peer data transmission technology, Skype... Read more
VueScan 9.4.41 - Scanner software with a...
VueScan is a scanning program that works with most high-quality flatbed and film scanners to produce scans that have excellent color fidelity and color balance. VueScan is easy to use, and has... Read more
Cloud 3.0.0 - File sharing from your men...
Cloud is simple file sharing for the Mac. Drag a file from your Mac to the CloudApp icon in the menubar and we take care of the rest. A link to the file will automatically be copied to your clipboard... Read more
LibreOffice 4.3.1.2 - Free Open Source o...
LibreOffice is an office suite (word processor, spreadsheet, presentations, drawing tool) compatible with other major office suites. The Document Foundation is coordinating development and... Read more

Latest Forum Discussions

See All

Alien Creeps TD Review
Alien Creeps TD Review By Jennifer Allen on September 2nd, 2014 Our Rating: :: EXPENSIVE DEFENSESUniversal App - Designed for iPhone and iPad Alien Creeps TD would be a fun if unremarkable Tower Defense game, but its heavy focus on... | Read more »
Function Space, a Social Network App for...
Function Space, a Social Network App for Science, Launches on iOS Posted by Ellis Spice on September 2nd, 2014 [ permalink ] | Read more »
Treasure Tombs: Ra Deal Coming from Bulk...
Treasure Tombs: Ra Deal Coming from Bulkypix and Dark Tonic This Fall Posted by Jessica Fisher on September 2nd, 2014 [ permalink ] Dark Tonic and | Read more »
Tiny Tower Vegas Review
Tiny Tower Vegas Review By Jennifer Allen on September 2nd, 2014 Our Rating: :: STEADY DEVELOPMENTUniversal App - Designed for iPhone and iPad Build a huge tower again but Vegas-style in Tiny Tower Vegas.   | Read more »
The Manhattan Project Review
The Manhattan Project Review By Andrew Fisher on September 2nd, 2014 Our Rating: :: ROCKET SCIENCEUniversal App - Designed for iPhone and iPad The Manhattan Project offers a great Euro-style gameplay experience, but it is totally... | Read more »
Rhonna Designs Magic (Photography)
Rhonna Designs Magic 1.0 Device: iOS Universal Category: Photography Price: $1.99, Version: 1.0 (iTunes) Description: Want to sprinkle *magic* on your photos? With RD Magic, you can add colors, filters, light leaks, bokeh, edges,... | Read more »
This Week at 148Apps: August 25-29, 2014
Shiny Happy App Reviews   | Read more »
Qube Kingdom – Tips, Tricks, Strategies,...
Qube Kingdom is a tower defense game from DeNA. You rally your troops – magicians, archers, knights, barbarians, and others – and fight against an evil menace looking to dominate your kingdom of tiny squares. Planning a war isn’t easy, so here are a... | Read more »
Qube Kingdom Review
Qube Kingdom Review By Nadia Oxford on August 29th, 2014 Our Rating: :: KIND OF A SQUARE KINGDOMUniversal App - Designed for iPhone and iPad Qube Kingdom has cute visuals, but it’s a pretty basic tower defense game at heart.   | Read more »
Fire in the Hole Review
Fire in the Hole Review By Rob Thomas on August 29th, 2014 Our Rating: :: WALK THE PLANKUniversal App - Designed for iPhone and iPad Seafoam’s Fire in the Hole looks like a bright, 8-bit throwback, but there’s not enough booty to... | Read more »

Price Scanner via MacPrices.net

Apple refurbished iPads available for up to $...
Apple is offering Certified Refurbished iPad Airs for up to $140 off MSRP. Apple’s one-year warranty is included with each model, and shipping is free. Stock tends to come and go with some of these... Read more
Are We Now In The Post-Post-PC Era?
A longtime and thoroughgoing laptop aficionado, I was more than a little dismayed by Steve Jobs’s declaration back in 2010 when he sprang the iPad on an unsuspecting world. that we’d entered a “post-... Read more
PC Outlook Improves, But 2014 Shipments Still...
According to the International Data Corporation (IDC) Worldwide Quarterly PC Tracker, worldwide PC shipments are expected to fall by -3.7 percent in 2014. To hat’s actually an improvement from the... Read more
IDC Lowers Tablet Sales Projections for 2014...
Following a second consecutive quarter of softer than expected demand, International Data Corporation (IDC) has lowered its worldwide tablet plus 2-in-1 forecast for 2014 to 233.1 million units. The... Read more
Apple now offering refurbished 21-inch 1.4GHz...
The Apple Store is now offering Apple Certified Refurbished 21″ 1.4GHz iMacs for $929 including free shipping plus Apple’s standard one-year warranty. Their price is $170 off the cost of new models,... Read more
Save $50 on the 2.5GHz Mac mini, on sale for...
B&H Photo has the 2.5GHz Mac mini on sale for $549.99 including free shipping. That’s $50 off MSRP, and B&H will also include a free copy of Parallels Desktop software. NY sales tax only. Read more
Save up to $300 on an iMac with Apple refurbi...
The Apple Store has Apple Certified Refurbished iMacs available for up to $300 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. These are the best prices on... Read more
The Rise of Phablets
Carlisle & Gallagher Consulting Group, a businesses and technology consulting firm focused solely on the financial services industry, has released an infographic depicting the convergence of... Read more
Bad Driver Database App Allows Good Drivers t...
Bad Driver Database 1.4 by Facile Group is a new iOS and Android app that lets users instantly input and see how many times a careless, reckless or just plain stupid driver has been added to the... Read more
Eddy – Cloud Music Player for iPhone/iPad Fre...
Ukraine based CapableBits announces the release of Eddy, its tiny, but smart and powerful cloud music player for iPhone and iPad that allows users to stream or download music directly from cloud... Read more

Jobs Board

*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
*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
*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
*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
Senior Event Manager, *Apple* Retail Market...
…This senior level position is responsible for leading and imagining the Apple Retail Team's global event strategy. Delivering an overarching brand story; in-store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.