I'm programming an application in Objective-C and I'm getting this error:
MyApp(2121,0xb0185000) malloc: *** error for object 0x1068310: double free
*** set a breakpoint in malloc_error_break to debug
It is happening when I release an NSAutoreleasePool and I can't figure out what object I'm releasing twice.
How do I set his breakpoint?
Is there a way to know what is this "object 0x1068310"?
When an object is "double-freed", the most common cause is that you're (unnecessarily) releasing an autoreleased object, and it is later autoreleased when the containing autorelease pool is emptied.
I've found that the best way to track down the extra release is to use the NSZombieEnabled environment variable for the affected executable in Xcode. For a quick rundown of how to use it, check out this CocoaDev wiki page. (In addition to this page, Apple has documented some incredibly obscure yet useful tips for debugging code in Xcode, some of which have saved my bacon more than a few times. I suggest checking out this Technical Note on developer.apple.com — link jumps to the section on Cocoa's Foundation framework).
Edit: You can often track the offending object down within the Xcode debugger, but it's often much easier if you use Instruments to assist you. From Xcode, choose Run → Start With Performance Tool → Object Allocations and you should be able to trace the offending object back to where it was created. (This will work best if you're enabled zombies as discussed above.) Note: Snow Leopard adds a Zombies tool to Instruments, accessible from the Run menu as well. Might be worth the $29 alone! ;-)
There is also a related SO question here.
You'll find out what the object is when you break in the debugger. Just look up the call stack and you will find where you free it. That will tell you which object it is.
The easiest way to set the breakpoint is to:
Go to Run -> Show -> Breakpoints (ALT-Command-B)
Scroll to the bottom of the list and add the symbol malloc_error_break
I just want to add my experience in addition to the answer of Quinn Taylor.
In one of my apps, I have to parse and save data into core data objects and later on get these objects to display on the views. In fact, the app works just fine and does not crash at all, until I tried to do a stress test of navigating back and forth multiple times, tried to open multiple views as fast as possible. The app crashes with the above message.
I have tried all the methods that Quinn suggested in his answer and still failed to find out where was the exact cause.
I set NSZombieEnabled=YES, and NSStackLogging=YES, ran the command shell malloc_history to find out why, but still no luck. It always points out to where I save the data into core data objects, in fact, I have checked thousand times the over released objects there, nothing odd.
Running in Instruments with various tools(Allocations, Leaks, etc...) still did not help. Enable the Guard Malloc still got nothing.
Final rescue: I tried to come back to the views where the objects were taken from Core Data and sent a retain message to all of these objects, and took note to these changes. It solved the issue!!!
So, I found out that I failed to retain one, that's exactly the cause. Just want to share my experience so you have another rescue for your app.
Open up the debugger console by pressing Cmd+Shift+R. There, type
break malloc_error_break
to set a breakpoint at the beginning of the malloc_error_break function.
If you want to find out what object is located at address 0x1068310, you can type the following into the debugger console:
print-object 0x1068310
Of course, you have to do this while the object is still alive -- if the object has already been freed by the time you do this, then this will not work.
Please find the below steps for how to find the object which is free and crash the application.
1) Click on the "Breakpoint navigator". 2) Then click on the
"+" button which is below. 3) Add the "Symbolic
Breakpoint..." from the list. 4) Add the
"malloc_error_break" keyword on the "Symbol" option.
Or you can also refer the below GIF presentation.
For me the issue was solved by
(gdb) call (void)_CFAutoreleasePoolPrintPools()
right after the crash. The address at the top of the stack was the address of the culprit. Threw in a retain and voila.
The address given in the log message did not get me anywhere. It never showed up in any of the various Instrumets. Apparently a pointer to some internal data which had already been freed.
Adding a symbolic breakpoint in Xcode 4
Just an update to make this relevant to Xcode 4...
From the Xcode 4 User Guide:
To add a symbolic breakpoint . . .
In the bottom-left corner of the breakpoint navigator, click the Add
button.
Choose Add Symbolic Breakpoint.
Enter the symbol name in the
Symbol field.
Click Done.
This is what the malloc_error_break breakpoint looks like in the Breakpoints window in Xcode.
Need to check the boxes to make it work.
alt text http://www.martijnthe.nl/wp-content/uploads/2009/08/Afbeelding-1.png
Check your classes and look under the dealloc method. Make sure you care calling [super dealloc].
I had this exact same problem and found out I was calling [self dealloc] instead. Just not paying attention.
In Xcode, click left of the line number to set a breakpoint. Then you can launch it by doing a "Build and Debug".
It is recommended to not have object that you create be autorelease since memory is a commodity on the iPhone. Apple recommends explicitly calling release.
To find these kinds of memory and pointer problems in general, you want to run your code against a runtime memory error checker like Valgrind. This should be able to point out lots of things your code is doing wrong, beyond those that cause it to crash.
Valgrind can work on OSX (though it says it's "unsupported and incomplete and buggy"), and with a little hacking someone got it to work on iPhone SDK executables.
Even better you can try Instruments, which is part of XCode. There's a tutorial for running it here.
If malloc_error_break is not helping...
The best way to solve this error is to run instruments with the NSZombies turned on. Instruments will flag you when the Zombie is messaged and you can trace directly back to the line of code.
Snow Leopard required, what a lifesaver though!
This is usually caused by some inspector, such as safari or safari preview. Refer to post or post and question.
Remove the select of AutoMatically Show Web ...., will remove this issue.
Note, just close safari or safari preview will not remove this issue. And you have to deselect both of safari and safari preview.
If this will not do, refer to this answer or post to debug it.
Related
I am trying to debug an EXC_BAD_ACCESS in my iPhone app. It is crashing on a method call and on the line of the method is EXC_BAD_ACCESS (code=1, address = xxx).
Before, I would have just used gdb info malloc-history <xxx> to start debugging, but I am having trouble finding a parallel command in LLDB.
I saw this thread that said to use Instruments, but when I do I still get the crash but I can't figure out how to tell exactly where the app is crashing from in Instruments.
I just need to figure out where this piece of memory that is crashing was pointing to. What is the best way to do this either using LLDB or Instruments?
You can see the malloc stack if you debug using instruments.
I encountered the same problem as you and similarly wanted to know how to get the malloc history when using lldb. Sadly I didn't find a nifty command like malloc-history found in gdb. To be honest I just switched my debugger over, but I found that annoying since I felt I shouldn't have to do that.
To find the malloc history using instruments:
Profile your project
Select Zombies from the list of instruments
Make your app trigger the problem
At this point you should be presented with the address that was already deallocated and you can explore it.
It should be a simple matter of viewing the malloc history at this point. I blacked out portions that had class / project names specific to the work I'm doing, but I think the essence and usefulness of how to go about getting this information is present.
A Last Word
The problem I ran into yielded a message like:
*** -[someClass retain]: message sent to deallocated instance 0x48081fb0 someProject(84051,0xacd902c0) malloc: recording malloc
stacks to disk using standard recorder
I was really puzzled where this retain was coming from since the code it was breaking on didn't have one (not in the getter or setter of the line it was on). It turns out that I was not calling removeObserver:forKeyPath: when a certain object was dealloc'ed. Later in execution KVO occurred do to a setter on a line and that blew up the program since KVO was trying to notify an object that was already released.
This problem is very easy to solve with an informative backtrace. Unfortunately with the latest version of iOS and Xcode, a good stack track is sometimes hard to come by. Fortunately you can set an 'Exception Breakpoint' in Xcode to allow you to examine this code prior to the EXC_BAD_ACCESS exception.
Open the breakpoint navigation in Xcode 4 (This looks like a rectangle with a point on the right side)
Press the '+' button at the bottom left and add an 'Exception Breakpoint'. Ensure you break 'On Throw' for 'All' exceptions.
Now you should get a full backtrace immediately prior to this exception occurring. This should allow you to at least zero in on where this exception is being thrown.
you can use command like this in lldb:
image lookup --address 0xec509b
you can find more commands at:LLDB TO GDB COMMAND MAP
Maybe is too late but for further assistance, on LLDB:
(lldb) p *(MyClassToPrint*)memory_address
E.g.
(lldb) p *(HomeViewController*)0x0a2bf700
I'm not exactly sure how to debug this but it seems that I'm getting EXEC BAD ACCESS in the main function of my app. There's no trace or logs of what's going on. I have NSZombiesEnabled but it doesn't seem to be showing anything. How would I debug this?
The first step in debugging is to find out where the crash happens. To do this you need to be able to reproduce the bug.
The Xcode debugger will usually tell you where the crash happened. It doesn't happen in main(), that's simply the entry point for the app and the top of the call stack. You can zoom in to the call stack with the slider at the bottom of the Debugger pane (in Xcode 4).
If the stack trace doesn't include any of your own code, then you may need to set an exception breakpoint in the Breakpoints pane and try again.
If all else fails, you'll have to do some digging:
1) Find a reproducible case where you can make the app crash every time (or almost every time) using the same actions.
2) Since you know what you're doing to make the app crash, you roughly know where in the code you should look. Set breakpoints at strategic places (or use NSLog). After some fishing, you'll find the exact line where the app crashed.
3) Fix the problem. :-)
It may be helpful.
Mainly you are getting this error because you have released something which is useful in furthur.So try to focus on the release.comment the line where you have used the release
Have you tried too clean and rebuild your project, and restart XCode. Latest versions of Xcode sometimes does EXEC BAD ACCESS int the main function with no reasons…
Turns out, pushing multiple view controllers simultaneously will cause this. Hard to debug that!
I'm still relatively new to XCode. I'm trying to build an iPhone application and after my application didFinishLaunchingWithOptions method runs but not inside of any code I can find I get this error:
[58036:207] *** -[_PFArray release]: message sent to deallocated instance 0x17cd2fe0
I have no idea what that means apart from the fact that some array somewhere is being released when it's already released?
The real problem, is that I have no clue how to use the information provided there to find where the error is. There is no line of code to look in, no variable name, etc.
How do I use this information to find what I'm looking for?
try enabling zombies, then xcode puts a breakpoint automatically to the line where this error occurs. Google nszombieenabled for more information
Edit: Well, I don't have much thing to do so here your guide,
Open instruments. It is an application and you can find it in "~/Developer/applications/instruments" (in a default installation path). Instruments will be your right hand so master it well.
In the opening screen, you will see a bunch of templates. Open the one named "Zombies". Note: You will often use the template named "Leaks". It shows all the memory leaks you forget to release.
From "choose target" (top left corner), select "choose target", and again select "choose target". From here, browse the project directory, and choose "build/Debug-iphonesimulator/executableName". Here I assume that you have built your project in debug profile with nszombieenabled to true. If not, build it so and reply 3.
Press record button (top left corner). The simulator will appear and your app will start. Then reproduce the error you have encountered.
Instruments will interrupt the app showing a message bubble. Do not close the bubble. From the bar in the middle of the window, select "objects list" instead of "statistics". Now, all the objects allocated up to this point lies in a list sorted by their memory addresses. Here scroll down to the memory address written inside the bubble. You can type the initial letters for fast scroll. Then double click the line you found. This brings you to the code where the object is allocated IF THE CODE IS YOURS. If it is not, then ios allocated the object. But do not worry, and double click the lines below and above to see if they are allocated by you. This should give you a hint where may be the problem is since these lines triggered ios to allocate the object (possibly).
This is all I can say. I hope you can find what causes this error.
My favorite candidate again. I am in the process of identifying memory leaks in my app (a puzzling challenge for a newbe like me).
I am using the xCode leak analyzer, but what puzzles me is how to trace back a memory leak to its variable or value. Is there a pointer to the instances that have reserved a memory address where a leak is identifyed?
How do I best go about it?
Are you aware of the Clang analyzer? Hit Cmd-Shift-A in Xcode and prepare to be pleasantly surprised. (This should run the Build → Build and Analyze command. It’s something like building the sources, but you get an analyzer log instead of the binary. Hopefully the analyzer will detect at least some of your leaks.)
The XCode leak analyzer tells you where the objects that were leaked were created, if you click the 'extended detail' tab on the right side of the toolbar at the bottom of the window. A call stack showing you which function created the object is shown on the right.
Most of the times when Iphone program crash, compiler show stack with full of no's, but these no's don't make any sense to me. Very rarely it point out where the problem might be and mostly there are these useless no's. How you can make sure that when your program crashes while development/testing, it shows at what place this cause this crash?
My iPhone dev life was horrible until I found NSZombieEnabled. By adding this flag into your executable, it will help you see any memory issues by letting you know what the name of the object that is at fault is.
This works by never actually releasing an object, but by wrapping it up as a "zombie" and setting a flag inside it that says it normally would have been released. This way, if you try to access it again, it still know what it was before you made the error, and with this little bit of information, you can usually backtrack to see what the issue was.
It especially helps in background threads when the Debugger sometimes craps out on any useful information.
VERY IMPORTANT TO NOTE however, is that you need to 100% make sure this is only in your debug code and not your distribution code. Because nothing is ever release, your app will leak and leak and leak. To remind me to do this, I put this log in my appdelegate:
if(getenv("NSZombieEnabled") || getenv("NSAutoreleaseFreedObjectCheckEnabled"))
NSLog(#"NSZombieEnabled/NSAutoreleaseFreedObjectCheckEnabled enabled!");
The key word you are looking for is "symbolicate". If you have a crash log from a device, you have to sun symbolicate on it in order to have the stack trace give you line numbers.
The function I have in my .profile to help me run the command is:
function desym
{
/Developer/Platforms/iPhoneOS.platform/Developer/Library/PrivateFrameworks/DTDeviceKit.framework/Versions/A/Resources/symbolicatecrash -A -v $1 | more
}
Basically you put the app bundle, the dsym file generated at build, and the crash log in the same directory and then run "dysm [CrashLog File Name]" to have the symbols correctly shown in the stack trace.
Note that it must be the same executable and dysm file that generated the crash! Every time you recompile, locations of things can change.
Things to do:
1) Debug with breakpoint on
2) Add a global breakpoint: objc_exception_throw
Then look in the Debugger window