Break on EXC_BAD_ACCESS in XCode?

Question

I'm new to iPhone development and XCode in general and have no idea how to begin troubleshooting an EXC_BAD_ACCESS signal. How can I get XCode to break at the exact line that is causing the error?

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I can't seem to get XCode to stop on the line causing the problem, but I do see the following lines in my debug console:

Sun Oct 25 15:12:14 jasonsmacbook TestProject[1289] : CGContextSetStrokeColorWithColor: invalid context

Sun Oct 25 15:12:14 jasonsmacbook TestProject[1289] : CGContextSetLineWidth: invalid context

Sun Oct 25 15:12:14 jasonsmacbook TestProject[1289] : CGContextAddPath: invalid context

Sun Oct 25 15:12:14 jasonsmacbook TestProject[1289] : CGContextDrawPath: invalid context

2009-10-25 15:12:14.680 LanderTest[1289:207] *** -[CFArray objectAtIndex:]: message sent to deallocated instance 0x3c4e610

Now, I am attempting to draw to the context I retrieve from UIGraphicsGetCurrentContext() and pass to the object that I want to draw with.

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Further trial and error debugging and I found that an NSMutableArray I have a property for on my class was a zombie. I went into the init function for the class and here's the code I was using:

if ((self = [super init])) {
     NSMutableArray *array = [NSMutableArray array];
     self.terrainBlocks = array;
     [array release];
    }
    return self; 
}

I removed the [array release] line and it no longer gives me the EXC_BAD_ACCESS signal, but I'm now confused about why this works. I thought that when I used the property, it automatically retained it for me, and thus I should release it from within init so that I don't have a leak. I'm thoroughly confused about how this works and all the guides and Stackoverflow questions I've read only confuse me more about how to set properties within my init method. There seems to be no consensus as to which way is the best.

Answer 1

For any EXC_BAD_ACCESS errors, you are usually trying to send a message to a released object. The BEST way to track these down is use NSZombieEnabled.

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 released, 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!");

If you need help finding the exact line, Do a Build-and-Debug (CMD-Y) instead of a Build-and-Run (CMD-R). When the app crashes, the debugger will show you exactly which line and in combination with NSZombieEnabled, you should be able to find out exactly why.

Answer 2

From the Stanford CS193P classes: if you add a breakpoint (manually, by editing breakpoints) for the symbolobjc_exception_throw you can get a much better picture of what went wrong - letting things proceed to the point where the debugger halts by itself tends to obscure things and screw up the stack trace. When you halt in objc_exception_throw you can often look back to exactly what access/operation caused your problem.

Answer 3

Xcode/gdb always breaks on EXC_BAD_ACCESS, you just need to work your way up the call stack to find the code that triggered it.

Note that these kinds of errors often occur with autoreleased objects, meaning that the ultimate cause of the problem won't be in the call stack that triggered EXC_BAD_ACCESS. That's when NSZombieEnabled and NSAutoreleaseFreedObjectCheckEnabled become helpful.

Answer 4

About your array. The line

NSMutableArray *array = [NSMutableArray array];

does not actually give you a retained object but rather an autorelease object. It probably gets retained in the next line but then you should not release it in the third line. See this

This is the fundamental rule:

You take ownership of an object if you create it using a method whose name begins with “alloc” or “new” or contains “copy” (for example, alloc, newObject, or mutableCopy), or if you send it a retain message. You are responsible for relinquishing ownership of objects you own using release or autorelease. Any other time you receive an object, you must not release it.

Answer 5

Another helpful approach is set breakpoints that will trigger directly after the exception occurs:

Open the breakpoints window (Run – Show – Breakpoints) and add two symbolic breakpoints called “objc_exception_throw” and “[NSException raise]"

From: http://blog.emmerinc.be/index.php/2009/03/19/break-on-exception-in-xcode/