How do I decide when I should release memory? Because If I release the memory, many times the application crashes.
You should release memory when you've finished with it. Objects are reference counted so, as long as you retain and release (and everyone else does too), you should have no problems.
If you find you're crashing because you release the memory, then either you or someone else isn't following the rules.
In either case, find out who isn't following the rules, and fix it. Don't hold on to memory just to avoid crashes. That way lies madness (and lack of memory).
Try the apple documentation on memory management in objective-c.
http://developer.apple.com/mac/library/documentation/cocoa/conceptual/MemoryMgmt/MemoryMgmt.html
Read Memory Management Programming Guide.
Related
I'm still trying to understand this piece of code that I found in a project I'm working on where the guy that created it left the company before I could ask.
This is the code:
-(void)releaseMySelf{
for (int i=myRetainCount; i>1; i--) {
[self release];
}
[self autorelease];
}
As far as I know, in Objective-C memory management model, the first rule is that the object that allocates another object, is also responsible to release it in the future. That's the reason I don't understand the meaning of this code. Is there is any meaning?
The author is trying to work around not understand memory management. He assumes that an object has a retain count that is increased by each retain and so tries to decrease it by calling that number of releases. Probably he has not implemented the "is also responsible to release it in the future." part of your understanding.
However see many answers here e.g. here and here and here.
Read Apple's memory management concepts.
The first link includes a quote from Apple
The retainCount method does not account for any pending autorelease
messages sent to the receiver.
Important: This method is typically of no value in debugging memory
management issues. Because any number of framework objects may have
retained an object in order to hold references to it, while at the
same time autorelease pools may be holding any number of deferred
releases on an object, it is very unlikely that you can get useful
information from this method. To understand the fundamental rules of
memory management that you must abide by, read “Memory Management
Rules”. To diagnose memory management problems, use a suitable tool:
The LLVM/Clang Static analyzer can typically find memory management
problems even before you run your program. The Object Alloc instrument
in the Instruments application (see Instruments User Guide) can track
object allocation and destruction. Shark (see Shark User Guide) also
profiles memory allocations (amongst numerous other aspects of your
program).
Since all answers seem to misread myRetainCount as [self retainCount], let me offer a reason why this code could have been written: It could be that this code is somehow spawning threads or otherwise having clients register with it, and that myRetainCount is effectively the number of those clients, kept separately from the actual OS retain count. However, each of the clients might get its own ObjC-style retain as well.
So this function might be called in a case where a request is aborted, and could just dispose of all the clients at once, and afterwards perform all the releases. It's not a good design, but if that's how the code works, (and you didn't leave out an int myRetainCount = [self retainCount], or overrides of retain/release) at least it's not necessarily buggy.
It is, however, very likely a bad distribution of responsibilities or a kludgey and hackneyed attempt at avoiding retain circles without really improving anything.
This is a dirty hack to force a memory release: if the rest of your program is written correctly, you never need to do anything like this. Normally, your retains and releases are in balance, so you never need to look at the retain count. What this piece of code says is "I don't know who retained me and forgot to release, I just want my memory to get released; I don't care that the others references would be dangling from now on". This is not going to compile with ARC (oddly enough, switching to ARC may just fix the error the author was trying to work around).
The meaning of the code is to force the object to deallocate right now, no matter what the future consequences may be. (And there will be consequences!)
The code is fatally flawed because it doesn't account for the fact that someone else actually "owns" that object. In other words, something "alloced" that object, and any number of other things may have "retained" that object (maybe a data structure like NSArray, maybe an autorelease pool, maybe some code on the stackframe that just does a "retain"); all those things share ownership in this object. If the object commits suicide (which is what releaseMySelf does), these "owners" suddenly point to bad memory, and this will lead to unexpected behavior.
Hopefully code written like this will just crash. Perhaps the original author avoided these crashes by leaking memory elsewhere.
Looking at the release notes of OS 10.7, there's some stuff about using Automatic Reference counting (ARC) to make memory management simple. I'm about to embark on a new Cocoa project soon and wondered whether it would be worth using the model (the way I understand it, you can't mix between using ARC and not using it)?
The dilemma seems to be using something new versus using something that could potentially save lots of debugging time later. But is that the case? Has anyone played around with it and found no real-world benefit?
Without a doubt you should use ARC. ARC injects the retain release calls at the most appropriate times, so you may actually see lower average memory use than you would if you didn't use ARC.
I have used ARC and it is immensely powerful. You stop having random crashes and your app just feels more responsive. As they said at WWDC, there is no reason not to use Automatic Reference Counting.
Also, you can use non-ARC files in the same project as ARC files.
I agree with FreeAsInBeer about using ARC. However, it should also be made clear that garbage collection (GC) should not be used. It looks like Apple is henceforth going to focus their efforts ARC, while letting GC die a slow death. This is wonderful because ARC is clearly a far superior technology, whereas GC is slow and extremly buggy.
So yes. Use ARC. Stay away from GC.
I'm looking a way to print all allocated objects when I get
applicationDidReceiveMemoryWarning
Is there someway I can see what objects are in memory. An system-api call, A framework, anything really.
I'm aware of Clang & debugging by code-inspection but I'm looking for a way to see at run-time why I have a memory leak.
Suggestions?
Thanks :)
Use the Instruments Allocations tool. You can navigate and see all live objects.
You can use instruments->leaks tool to see where you have a leak. It's quite precise and you can see where you allocated the object.
I don't know of a way to do this at runtime without keeping track of all the objects yourself. If you're doing this during debugging, you can pause execution at that point and take a "heapshot" of your process using Instruments. That will show you everything that's been allocated on the heap.
I'm thinking of writing an app for the iphone. But I'm confused in regards to the objective-c language. Am I correct in saying that I have to do my own memory management? Is Objective-C a managed language such as C# and/or Java?
Thank you!
"Managed" is a marketing term that has no technical meaning. Objective C for the iPhone is not garbage collected, but uses retain counting memory management. So in some sense, you have to do your own memory management, but it's limited to knowing the Cocoa retain counting methodology.
Edit: My comments on "managed" refer to a previous post version. The comment is left here because I'm tired of seeing the word.
There is no garbage collection feature. However, any object you copy, retain, alloc etc. has its retain count bumped up by one, and you are responsible for calling a releaseor an autorelease - you own an instance of that object. If you don't call [<YOUROBJECT> release]; or [<YOUROBJECT> autorelease];, the object remains in the iPhone's/iPod Touch's/iPad's memory, but the pointer no longer remains, and a memory leak occurs, as that bit of memory can no longer be accessed. Autoreleasing adds the object to the autorelease pool, and means that it will become released at some point in the future. Always use a standard release where possible, as autorelease means that it will still stay in the device's memory for a while, you can never be sure when it will be released, and it is a slightly more demanding method to call.
Never call dealloc on an object - releasing or autoreleasing is sufficient, and if required, the object will automatically be dealloced.
Make sure you never release objects you do not own, and if you do release objects, you release them after you no longer have any use of them - otherwise, you may try to access that object, and the device cannot find the object, and the situation results in your app crashing due to an EXEC_BAC_ACCESS error - to find out the root of the problem, open Instruments, add the Object Allocation instrument and enable NSZombie detection in its preferences (or add the Zombies instrument (only available in iPhone SDK 3.2+)). You can then view the entire history of the object which causes the problem, and jump to the exact line of code that caused the problem.
You can read more here.
Hope this helped
As others have pointed out Objective-C has no garbage collector on the iPhone, but it does have one for Mac OSX. Here is an article describing this in more detail: http://vasudevkamath.blogspot.com/2010/01/objective-c-my-opinions.html
You have to do your own memory management with Objective-C on the iPhone.
The answer isn't specific to Objective-C. If you're doing iPhone development, no...there is no garbage collector. If you're developing for Mac OSX, however, there is garbage collection.
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.