1.When my app crashes and there's no message in console window Xcode shows me a green line with signature EXC_BAD_ACCESS or PROGRAM RECEIVED SIGNAL SIGABRT in the code editor. The question is: does Xcode always put this green line at the line of code where a bug is placed? If i see this green line put at the line
[myObject myMethod];
may i be confident that the bug should be searched for inside the myMethod function? Or maybe it may mean something else?
2.Sometimes this green line is put inside main.m file at the line
int retVal = UIApplicationMain(argc, argv, nil, nil);
Do you know what specific about placing the hint there? What does it mean?
3.Also sometimes the strangest thing happens: Xcode shows me a file with assembler code and the green line inside this code. Why? What should i understand when i see that?
4.If i repeatedly do the following: enter a screen, then do something, then quit the screen - my app crashes. It may crash at the second time or at the seventh one. What is the most common causes for such a crash? A memory leak? But Analyze tells me there are no leaks in my app. What do you think it may be?
As I said in another post :
In XCode, go to menu "edit scheme", choose the running configuration and add 'NSZombieEnabled' like in the picture below, when your apps crashes, it will provide you additional infos on the crash that should help you debug it.
Note that when your application debug is over, remove the NSZombieEnabled command as it impacts the application performances
The simple answer to point 4: The most common cause of a crash accessing deallocated/over-released memory.
You can check for NSZombies or debug any memory references that you can find in the error message.
You can find some hints here iPhone Xcode debugging
Regarding (3) I do not think there is a way to interpret that assembly code.
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!
When I usually get runtime errors I can open up the Debugger view in XCode and see what line it occurs and fix accordingly. What do I do when there is no line number? I must have an allocation/deallocation memory error somewhere, but how do I go about finding it? The instruments tool is virtually unuseable and slow - sometimes not loading up at all. I have attached the screenshot of my Debugger window: http://www.freeimagehosting.net/image.php?1c4089f3e4.png.
alt text http://www.freeimagehosting.net/image.php?1c4089f3e4.png
According to that stacktrace you have an over-release. Use the Zombies instrument or set the NSZombieEnabled environment variable to YES. This will let you know what object is being over-released.
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
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.