Xcode 4 Debugging Crash-Course

On October 6, 2011, in idevblogaday, by Steffen Itterheim

What do you do if your app doesn’t behave as it should, or even crashes?

Answer A: Post your problem in just about every programming forum.
Answer B: Use the Xcode Debugger to analyze what’s going wrong.

Since most of you already know how to do A I’ll focus on B in this Xcode 4 Debugging Crash-Course. It’s kind of aimed at beginning Xcode developers but that’s just because I hope – against better knowledge – that experienced developers already know that … thing … that debugger stuff. Ya know?

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This Kobold2D FAQ article explains the difference between Corona SDK and iPhone Wax library, and evaluates the existing and future options for Lua scripting in Kobold2D.

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The Art of Assertion (as it pertains to Xcode)

On November 9, 2010, in Programming, Xcode, by Steffen Itterheim

Recently, I’ve changed all my project’s source code files from .m to .mm file extensions. This is telling the compiler to compile the files as Objective-C++/C++ code instead of the default Objective-C/C. I needed to do so because I’m using Box2D, and as a habit I intend to use .mm from now on for every source file I create.

However, beware the subtle differences. Previously, if an NSAssert was triggered, it halted the program execution. But in .mm files it simply prints the assertion message to the console while the App continues execution. This leads to overlooked assertions, or continuously dumping assertion messages to the console. Clearly not what I wanted. Changing the file extension from .mm back to .m fixed the problem, but that’s not an option I have for all files.

I looked around and found a blog article by Vincent Gable mentioning that NSAssert is considered harmful. That caught my attention, because it described the same problem that I experienced (NSAssert not halting execution) while basing the argument on Voodoo (“Sometimes it does, sometimes it does not …”) respectively no argument at all, it’s just telling you “NSAssert is unreliable, therefore dangerous”.

It’s as if he wants me to prove him wrong. :)

In Vincent’s defense, the wrong he’s done unto NSAssert he makes up for by having written a great logging method which I’m going to add to gocos because it’s so handy.

Why assert() is less useful than NSAssert

First of all, the assert() function has one big problem: it doesn’t allow parameters to be added to the output string, since the output string can only be a constant string, like in this example:

[cc lang=”objc”]
assert(value < maxValue && @"value too big!"); [/cc] With NSAssert you can actually embedd some values into the assertion message: [cc lang="objc"] NSAssert2(value < maxValue, @"value %i too big (max:%i)!", value, maxValue); [/cc] It is tremendeously helpful in many cases to actually see the offending or interesting values in the error message, without having to fire up the debugger. Especially when it comes to filenames, and even more so if end users might see those assertions. Not a problem on the iOS, but think of developing desktop Apps.

Why NSAssert isn’t harmful, and how to fix it

So then, why did my App not stop execution when an NSAssert triggered? I can’t really answer you that, Apple’s docs say that NSAssert will raise an NSInternalInconsistencyException. So that should stop the App from running? It does not seem to be the case when you’re using the Objective-C++ compiler, and I’m not running the code on a different thread either. Maybe someone can shed some light as to why NSAssert in C++ code doesn’t automatically stop the App from running.

The good thing is, there’s an easy way to fix that. In Xcode choose Run -> Show -> Breakpoints from the menu, to bring up the breakpoints list. In the breakpoint list you only need to add the symbol objc_exception_throw. Next time an NSAssert triggers, the App will stop immediately and bring up the debugger.

Some places (for example: here) will also tell you to add [NSException raise] to the breakpoints list. That’s not necessary, this has been replaced with objc_exception_throw since Mac OS X 10.5.

Turning off assertions

I must admit, I’m totally spoiled by Visual Studio. Why Xcode requires you to add NS_BLOCK_ASSERTIONS as a macro to your project’s build settings in order to not compile NSAssert in release builds is beyond me. If you’re using NSAssert in your code, make sure that your release and/or distribution builds define the NS_BLOCK_ASSERTIONS macro.

If you do use the assert() macro however, or if you’re using a 3rd party library that uses assert(), make sure to also define NDEBUG for release/distribution builds.

As a side note, when I update my cocos2d-project respectively replace it with gocos, it will have all these settings properly configured.

Catching uncaught exceptions

On a related matter, there’s always the issue of uncaught exceptions simply halting your App, with little chance to debug the actual cause. It happens rarely but when it does, it would be really helpful to also bring up the debugger with the current call stack and everything. You can set a global exception handler to catch uncaught exceptions by calling the NSSetUncaughtExceptionHandler in the applicationDidFinishLaunching method of your AppDelegate:

[cc lang=”objc”]
void onUncaughtException(NSException* exception)
{
NSLog(@”uncaught exception: %@”, exception.description);
}

-(void) applicationDidFinishLaunching:(UIApplication*)application
{
NSSetUncaughtExceptionHandler(&onUncaughtException);

}
[/cc]

In the onUncaughtException method you’ll simply log the exception and (very important) add a breakpoint. The NSLog message serves the purpose of being able to easily set a breakpoint inside this method. It should not be another NSAssert because that will throw another exception, which will only serve to make debugging more complicated when a simple breakpoint suffices.