I'm studying Objective-C. I've found out about the ARC. I made a simple program with one class and one instance variable NSUInteger.
For educational purposes I'd like to examine the memory allocation/deallocation while the program is running.
Are there any console tools to see a program's memory? Or may be it is possible to do it in the Xcode itself? In other words I'd like to see the memory snapshot in different points in times when an object was allocated and then an object was deallocated.
Thank you.
First, NSUInteger variables are no objects in the meaning of Objective-C. They are handled with the C memory management. (Typically they are local vars on the stack, freed, when the local scope is left.)
So let's go to real instance objects of classes like NSNumber or NSString or – more important – MyCustomClass. You can see the whole processing of memory management, when you write a class and compile that with manual memory management. (This is possible via the compiler options. Select the project, go to build phases/compile sources and you will find an extra column compiler flags.)) Since ARC and MRC work together, ARC code will execute the MRC memory handling methods. Simply overwrite them in the MRC class and do some logging, set some breakpoints and so on.
But, this is very important, as long as you only deal with ARC code, you simply do not have to care about memory management. Maybe it is a good idea to learn that, but it is not something you have to do necessarily.
Related
Coming from the world of managed memory, wondering what would be the proper way to clean up objects when using ARC.
For example: if declaring an instance variable in C#, .NET will allow the GC to pick it up once it leaves scope (method/loop body, etc)
What's the proper way to clean-up in Objective-C? Just set the reference/pointer to nil or call dealloc or will ARC detect that no external references are pointing to the instance once execution leaves scope and do the job for you?
ARC means "Automatic Reference Counting" and is just a way to let the compiler add the calls to retain/release/autorelease for you. It's not the same as GC but in most cases, you can consider that objects lifetime is automatically managed for you, like in GC.
If you want more information, you should read LLVM document on ARC
Last note: never call dealloc yourself. dealloc is the object's finalizer which is called once the ObjC runtime determines that the object reference count has reached 0. This method is only meant to be overriden by subclasses. In ARC mode, you generally don't need to do that, except if your object references non-object ivars that need to be finalized once the object itself is finalized.
will ARC detect that no external references are pointing to the
instance once execution leaves scope and do the job for you
Basically, yes, that's exactly what ARC will do. You don't need to clean up objects when you're using ARC; in fact, you can't (it stops you from trying to perform manual memory management).
You might want to consult the relevant discussion in my book:
http://www.apeth.com/iOSBook/ch12.html#_memory_management
It explains what's really happening behind the scenes (how memory is actually managed) and then goes on to describe how ARC shields you from most of it.
Note that (as I explain in the URL referenced above) it mostly isn't done by anything like garbage collection: it's done by inserting invisible explicit memory management throughout your code.
Well, in the past, iOS programmers were responsible for telling the system when they were done using an object that they allocated by sending the object a release message. That was done in accordance with a memory management system known as manual reference counting. As of Xcode 4.2, programmers no longer have to worry about this and can rely on the system to take care of releasing memory as necessary. This is done through a mechanism known as Automatic Reference Counting, or ARC for short. ARC is enabled by default when you compile new applications using Xcode 4.2 or later.
You can also disable ARC, in your Xcode interface, go to your main project (not main.h) your actual Xcode project, and select it, you should see a window in Xcode that displays the settings for your project, there will be one that says 'Objective-C Automatic Reference Counting' and it will be set to 'Yes', deactivate it (to 'No') and you shouldn't worry about the ARC, if you come from the world of data management and memory as you said, but keep in mind that it would be easier to you to keep updated to the iOS new features system, that are easier to the programmer to program, it just makes our life easier.
And now, the 'proper way to clean-up in Xcode' with ARC is with 'alloc' and 'init'.
With ARC in Xcode you do not need to worry for 'cleaning' that's the job of Xcode now, you just need to:
1) Create a variable.
2) Allocate.
3) Initialize.
That's it.
An example here:
int main (int argc, char * argv[])
{
#autoreleasepool {
Variable *myVariable;
// Create an instance of a Variable and initialize it
myVariable = [Variable alloc];
myVariable = [myVariable init];
// Set variable to 4/20
[myVariable setNumerator: 4];
[myVariable setDenominator: 20];
// Display the variable using the print method
NSLog (#"The value of myVariable is:");
[myVariable print];
}
return 0;
}
Just allocate and then initialize, yo do not need to do any thing else.
Keep in mind getters and setters.
Recently I've begun to code in Objective-C for iOS 5 devices. My brand new MacBook is loaded with Xcode 4.2 and the latest Mac & iOS SDKs. So far it's been a fun experience but there is one problem that I see with the current state of documentation and available books.
Specifically, most books (that have yet to be updated) always reference how and when to manage your memory. This is great, however, the current SDK/compiler includes Automatic Reference Counting and since I leave this turned on for my projects, I have no clue as to what I should personally monitor and manage myself.
I come from a C# background. Memory management in C# (technically, .NET) is entirely handled by the framework garbage collector. I understand that ARC is actually a compiler feature that automatically adds boiler-plate code where it belongs. Furthermore, my attempts to try and discover where I should manage my own releasing of objects has caused nothing but compiler errors because ARC wants to take care of it for me.
I have yet to find a case where I've needed to manage my objects. I am becoming "lazy" because I don't know what to monitor and release myself and I am completely oblivious about how this behavior could affect the performance of my application.
In new-user terms, what "gotchas" should I be aware of while using ARC in my iOS projects? I've read a few questions regarding memory management and ARC around here but, to be honest, they are not to friendly to the new iOS developer. Could someone please give a reasonable, bullet-point list that explains what problems and issues to watch out for, as well as a fair guide as to when self-management of memory is necessary?
Circular References. When objects are codependent, they will leak. You will need to mark some references as weak, and Instruments can help you locate these references. These leaks won't even show up as leaks because they hold strong references to each other.
Create Autorelease Pools #autorelease to keep autorelease pool sizes down where you create many autoreleased objects (directly or indirectly). Specifically, your program and programs you depend on will autorelease many objects (ARC or otherwise). An autoreleased object is one which will be released "in the future". Every Cocoa program expects an autorelease pool to exist on each thread. This is why you create a new pool when you create a new thread, and why you create one in main. The pools operate as a stack - you may push and pop pools. When a pool is destroyed it sends its deferred release message to every object it holds. This means that really large loops with many temporary allocations may result in many objects which are referenced only by the pool, and the pool may grow very large. For this reason, you drain manage pools directly in some cases to minimize the number of objects that are waiting to be released and deallocated.
Use proper bridging/casting. Sometimes you will need to manage lifetimes explicitly. ARC handles the obvious cases, but there are complex cases where you will need to manage lifetimes explicitly.
When using malloc'ed and new'ed allocations, as well as opaque types in 'Core' APIs. ARC only manages NSObject types. You still need to explicitly free, delete, and use correct ref counting for these allocations, types, and when interfacing with those APIs.
Always follow NS-API naming conventions, and avoid explicit memory management attributes where possible.
You'll obviously need MRC when you compile sources without ARC or GC. This is quite common when using/working with other libraries/code bodies. Of course, the compiler handles interactions correctly so your ARC program should not leak.
ARC handles most of what you will need if you use proper naming and written style, but there will be a few other corner cases. Fortunately, you can still run Leaks and Zombies to locate these issues if you don't realize them during development.
Can someone briefly explain to me how ARC works? I know it's different from Garbage Collection, but I was just wondering exactly how it worked.
Also, if ARC does what GC does without hindering performance, then why does Java use GC? Why doesn't it use ARC as well?
Every new developer who comes to Objective-C has to learn the rigid rules of when to retain, release, and autorelease objects. These rules even specify naming conventions that imply the retain count of objects returned from methods. Memory management in Objective-C becomes second nature once you take these rules to heart and apply them consistently, but even the most experienced Cocoa developers slip up from time to time.
With the Clang Static Analyzer, the LLVM developers realized that these rules were reliable enough that they could build a tool to point out memory leaks and overreleases within the paths that your code takes.
Automatic reference counting (ARC) is the next logical step. If the compiler can recognize where you should be retaining and releasing objects, why not have it insert that code for you? Rigid, repetitive tasks are what compilers and their brethren are great at. Humans forget things and make mistakes, but computers are much more consistent.
However, this doesn't completely free you from worrying about memory management on these platforms. I describe the primary issue to watch out for (retain cycles) in my answer here, which may require a little thought on your part to mark weak pointers. However, that's minor when compared to what you're gaining in ARC.
When compared to manual memory management and garbage collection, ARC gives you the best of both worlds by cutting out the need to write retain / release code, yet not having the halting and sawtooth memory profiles seen in a garbage collected environment. About the only advantages garbage collection has over this are its ability to deal with retain cycles and the fact that atomic property assignments are inexpensive (as discussed here). I know I'm replacing all of my existing Mac GC code with ARC implementations.
As to whether this could be extended to other languages, it seems geared around the reference counting system in Objective-C. It might be difficult to apply this to Java or other languages, but I don't know enough about the low-level compiler details to make a definitive statement there. Given that Apple is the one pushing this effort in LLVM, Objective-C will come first unless another party commits significant resources of their own to this.
The unveiling of this shocked developers at WWDC, so people weren't aware that something like this could be done. It may appear on other platforms over time, but for now it's exclusive to LLVM and Objective-C.
ARC is just play old retain/release (MRC) with the compiler figuring out when to call retain/release. It will tend to have higher performance, lower peak memory use, and more predictable performance than a GC system.
On the other hand some types of data structure are not possible with ARC (or MRC), while GC can handle them.
As an example, if you have a class named node, and node has an NSArray of children, and a single reference to its parent that "just works" with GC. With ARC (and manual reference counting as well) you have a problem. Any given node will be referenced from its children and also from its parent.
Like:
A -> [B1, B2, B3]
B1 -> A, B2 -> A, B3 -> A
All is fine while you are using A (say via a local variable).
When you are done with it (and B1/B2/B3), a GC system will eventually decide to look at everything it can find starting from the stack and CPU registers. It will never find A,B1,B2,B3 so it will finalize them and recycle the memory into other objects.
When you use ARC or MRC, and finish with A it have a refcount of 3 (B1, B2, and B3 all reference it), and B1/B2/B3 will all have a reference count of 1 (A's NSArray holds one reference to each). So all of those objects remain live even though nothing can ever use them.
The common solution is to decide one of those references needs to be weak (not contribute to the reference count). That will work for some usage patterns, for example if you reference B1/B2/B3 only via A. However in other patterns it fails. For example if you will sometimes hold onto B1, and expect to climb back up via the parent pointer and find A. With a weak reference if you only hold onto B1, A can (and normally will) evaporate, and take B2, and B3 with it.
Sometimes this isn't an issue, but some very useful and natural ways of working with complex structures of data are very difficult to use with ARC/MRC.
So ARC targets the same sort of problems GC targets. However ARC works on a more limited set of usage patterns then GC, so if you took a GC language (like Java) and grafted something like ARC onto it some programs wouldn't work any more (or at least would generate tons of abandoned memory, and may cause serious swapping issues or run out of memory or swap space).
You can also say ARC puts a bigger priority on performance (or maybe predictability) while GC puts a bigger priority on being a generic solution. As a result GC has less predictable CPU/memory demands, and a lower performance (normally) than ARC, but can handle any usage pattern. ARC will work much better for many many common usage patterns, but for a few (valid!) usage patterns it will fall over and die.
Magic
But more specifically ARC works by doing exactly what you would do with your code (with certain minor differences). ARC is a compile time technology, unlike GC which is runtime and will impact your performance negatively. ARC will track the references to objects for you and synthesize the retain/release/autorelease methods according to the normal rules. Because of this ARC can also release things as soon as they are no longer needed, rather than throwing them into an autorelease pool purely for convention sake.
Some other improvements include zeroing weak references, automatic copying of blocks to the heap, speedups across the board (6x for autorelease pools!).
More detailed discussion about how all this works is found in the LLVM Docs on ARC.
It varies greatly from garbage collection. Have you seen the warnings that tell you that you may be leaking objects on different lines? Those statements even tell you on what line you allocated the object. This has been taken a step further and now can insert retain/release statements at the proper locations, better than most programmers, almost 100% of the time. Occasionally there are some weird instances of retained objects that you need to help it out with.
Very well explained by Apple developer documentation. Read "How ARC Works"
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To make sure that instances don’t disappear while they are still needed, ARC tracks how many properties, constants, and variables are currently referring to each class instance. ARC will not deallocate an instance as long as at least one active reference to that instance still exists.
To know Diff. between Garbage collection and ARC: Read this
ARC is a compiler feature that provides automatic memory management of objects.
Instead of you having to remember when to use retain, release, and autorelease, ARC evaluates the lifetime requirements of your objects and automatically inserts appropriate memory management calls for you at compile time. The compiler also generates appropriate dealloc methods for you.
The compiler inserts the necessary retain/release calls at compile time, but those calls are executed at runtime, just like any other code.
The following diagram would give you the better understanding of how ARC works.
Those who're new in iOS development and not having work experience on Objective C.
Please refer the Apple's documentation for Advanced Memory Management Programming Guide for better understanding of memory management.
Sorry for long description, however the questions aren't so easy...
My project written without GC. Recently I found a memory leak that I can't find. I did use new Xcode Analyzer without a result. I did read my code line by line and verified all alloc/release/copy/autorelease/mutableCopy/retain and pools... - still nothing.
Preamble: Standard Instruments and Omni Leak Checker don't work for me by some reason (Omin Tool rejects my app, Instruments.app (Leaks) eats too many memory and CPU so I have no chance to use it).
So I wanna write and use my own code to hook & track "all" alloc/allocWithZone:/dealloc messages statistics to write some simple own leaks checking library (the main goal is only to mark objects' class names with possible leaks).
The main hooking technique that I use:
Method originalAllocWithZone = class_getClassMethod([NSObject class],#selector(allocWithZone:));
if (originalAllocWithZone)
{
imp_azo = (t_impAZOriginal)method_getImplementation(originalAllocWithZone);
if (imp_azo)
{
Method hookedAllocWithZone = class_getClassMethod([NSObject class],#selector(hookedAllocWithZone:));
if (hookedAllocWithZone)
{
method_setImplementation(originalAllocWithZone,method_getImplementation(hookedAllocWithZone));
fprintf(stderr,"Leaks Hook: allocWithZone: ; Installed\n");
}
}
}
code like this for hook the alloc method, and dealloc as NSObject category method.
I save IMP for previous methods implementation then register & calculate all alloc/allocWithZone: calls as increment (+1) stat-array NSInteger values, and dealloc calls as decrement (-1).
As end point I call previous implementation and return value.
In concept all works just fine.
If it needs, I can even detect when class are part of class cluster (like NSString, NSPathStore2; NSDate, __NSCFDate)... via some normalize-function (but it doesn't matter for the issues described bellow).
However this technique has some issues:
Not all classes can be caught, for
example, [NSDate date] doesn't catch
in alloc/allocWithZone: at all, however, I can see alloc call in GDB
Since I'm trying to use auto singleton detection technique (based on retainCount readind) to auto exclude some objects from final statistics, NSLocale creation freezes on pre-init stage when starting of full Cocoa application (actually, even simple Objective-C command line utility with the Foundation framework included has some additional initialization before main()) - by GDB there is allocWithZone: calls one after other,....
Full Concept-Project draft sources uploaded here: http://unclemif.com/external/DILeak.zip (3.5 Kb)
Run make from Terminal.app to compile it, run ./concept to show it in action.
The 1st Question: Why I can't catch all object allocations by hooking alloc & allocWithZone: methods?
The 2nd Question: Why hooked allocWithZone: freezes in CFGetRetainCount (or [inst retainCount]) for some classes...
Holy re-inventing the wheel, batman!
You are making this way harder than it needs to be. There is absolutely no need whatsoever to roll your own object tracking tools (though it is an interesting mental exercise).
Because you are using GC, the tools for tracking allocations and identifying leaks are all very mature.
Under GC, a leak will take one of two forms; either there will be a strong reference to the object that should long ago been destroyed or the object has been CFRetain'd without a balancing CFRelease.
The collector is quite adept at figuring out why any given object is remaining beyond its welcome.
Thus, you need to find some set of objects that are sticking around too long. Any object will do. Once you have the address of said object, you can use the Object Graph instrument in Instruments to figure out why it is sticking around; figure out what is still referring to it or where it was retained.
Or, from gdb, use info gc-roots 0xaddr to find all of the various things that are rooting the object. If you turn on malloc history (see the malloc man page), you can get the allocation histories of the objects that are holding the reference.
Oh, without GC, huh...
You are still left with a plethora of tools and no need to re-invent the wheel.
The leaks command line tool will often give you some good clues. Turn on MallocStackLoggingNoCompact to be able to use malloc_history (another command line tool).
Or use the ObjectAlloc instrument.
In any case, you need to identify an object or two that is being leaked. With that, you can figure out what is hanging on to it. In non-GC, that is entirely a case of figuring out why it there is a retain not balanced by a release.
Even without the Leaks instrument, Instruments can still help you.
Start with the Leaks template, then delete the Leaks instrument from it (since you say it uses too much memory). ObjectAlloc alone will tell you all of your objects' allocations and deallocations, and (with an option turned on, which it is by default in the Leaks template) all of their retentions and releases as well.
You can set the ObjectAlloc instrument to only show you objects that still exist; if you bring the application to the point where no objects (or no objects of a certain class) should exist, and such objects do still exist, then you have a leak. You can then drill down to find the cause of the leak.
This video may help.
Start from the Xcode templates. Don't try to roll your own main() routine for a cocoa app until you know what you're doing.
Memory (and resource) leaks happen. How do you make sure they don't?
What tips & techniques would you suggest to help avoid creating memory leaks in first place?
Once you have an application that is leaking how do you track down the source of leaks?
(Oh and please avoid the "just use GC" answer. Until the iPhone supports GC this isn't a valid answer, and even then - it is possible to leak resources and memory on GC)
In XCode 4.5, use the built in Static Analyzer.
In versions of XCode prior to 3.3, you might have to download the static analyzer. These links show you how:
Use the LLVM/Clang Static Analyzer
To avoid creating memory leaks in the first place, use the Clang Static Analyzer to -- unsurprisingly -- analyse your C and Objective-C code (no C++ yet) on Mac OS X 10.5. It's trivial to install and use:
Download the latest version from this page.
From the command-line, cd to your project directory.
Execute scan-build -k -V xcodebuild.
(There are some additional constraints etc., in particular you should analyze a project in its "Debug" configuration -- see http://clang.llvm.org/StaticAnalysisUsage.html for details -- the but that's more-or-less what it boils down to.)
The analyser then produces a set of web pages for you that shows likely memory management and other basic problems that the compiler is unable to detect.
If your project does not target Mac OS X desktop, there are a couple of other details:
Set the Base SDK for All Configurations to an SDK that uses the Mac OS X desktop frameworks...
Set the Command Line Build to use the Debug configuration.
(This is largely the same answer as to this question.)
Don't overthink memory management
For some reason, many developers (especially early on) make memory management more difficult for themselves than it ever need be, frequently by overthinking the problem or imagining it to be more complicated than it is.
The fundamental rules are very simple. You should concentrate just on following those. Don't worry about what other objects might do, or what the retain count is of your object. Trust that everyone else is abiding by the same contract and it will all Just Work.
In particular, I'll reiterate the point about not worrying about the retain count of your objects. The retain count itself may be misleading for various reasons. If you find yourself logging the retain count of an object, you're almost certainly heading down the wrong path. Step back and ask yourself, are you following the fundamental rules?
Always use accessor methods; declare accessors using properties
You make life much simpler for yourself if you always use accessor methods to assign values to instance variables (except in init* and dealloc methods). Apart from ensuring that any side-effects (such as KVO change notifications) are properly triggered, it makes it much less likely that you'll suffer a copy-and-paste or some other logic error than if you sprinkle your code with retains and releases.
When declaring accessors, you should always use the Objective-C 2 properties feature. The property declarations make the memory management semantics of the accessors explicit. They also provide an easy way for you to cross-check with your dealloc method to make sure that you have released all the properties you declared as retain or copy.
The Instruments Leaks tool is pretty good at finding a certain class of memory leak. Just use "Start with Performance Tool" / "Leaks" menu item to automatically run your application through this tool. Works for Mac OS X and iPhone (simulator or device).
The Leaks tool helps you find sources of leaks, but doesn't help so much tracking down the where the leaked memory is being retained.
Follow the rules for retaining and releasing (or use Garbage Collection). They're summarized here.
Use Instruments to track down leaks. You can run an application under Instruments by using Build > Start With Performance Tool in Xcode.
I remember using a tool by Omni a while back when I was trying to track down some memory leaks that would show all retain/release/autorelease calls on an object. I think it showed stack traces for the allocation as well as all retains and releases on the object.
http://www.omnigroup.com/developer/omniobjectmeter/
First of all, it's vitally important that your use of [ ] and { } brackets and braces match the universal standard. OK, just kiddin'.
When looking at leaks, you can assume that the leak is due to a problem in your code but that's not 100% of the fault. In some cases, there may be something happening in Apple's (gasp!) code that is at fault. And it may be something that's hard to find, because it doesn't show up as cocoa objects being allocated. I've reported leak bugs to Apple in the past.
Leaks are sometimes hard to find because the clues you find (e.g. hundreds of strings leaked) may happen not because those objects directly responsible for the strings are leaking, but because something is leaking that object. Often you have to dig through the leaves and branches of a leaking 'tree' in order to find the 'root' of the problem.
Prevention: One of my main rules is to really, really, really avoid ever allocating an object without just autoreleasing it right there on the spot. Anywhere that you alloc/init an object and then release it later on down in the block of code is an opportunity for you to make a mistake. Either you forget to release it, or you throw an exception so that the release never gets called, or you put a 'return' statement for early exit somewhere in the method (something I try to avoid also).
You can build the beta port of Valgrind from here: http://www.sealiesoftware.com/valgrind/
It's far more useful than any static analysis, but doesn't have any special Cocoa support yet that I know of.
Obviously you need to understand the basic memory management concepts to begin with. But in terms of chasing down leaks, I highly recommend reading this tutorial on using the Leaks mode in Instruments.