One element of making a solid application is to make it as easy as possible for users to report problems to you. But, it doesn't help you the developer much if you get a report such as "I received an error 42 when I tried to load my file." Wouldn't it be better to have the error report tell you the exact line of code.

I'm going to show you how to extract a stack trace from an NSException. Apple has kindly posted a guide to printing symbolic stack traces, however, their solution requires that the Developer Tools be installed because it uses the atos command-line utility.

The method I will outline below does not require any external command-line tools which may-or-may-not be installed. Instead, they require a couple of glibc functions.

And, just to get fancy, this code will also de-mangle C++ symbols in your stack traces. Those who are working in Objective-C only (lucky bastards) can delete half of the code with no repercussions.

This is the sort of problem best described with code, so here goes:

NSString * UTstackTraceForException(NSException * exception)
{
    NSArray * addresses = [exception callStackReturnAddresses];
    void * backtrace_frames[[addresses count]];
    int i = 0;
    NSString * trace = @"";
    NSString * frame_description, * symbol;

    for (NSNumber * address in addresses)
    {
        backtrace_frames[i] = (void *)[address unsignedLongValue];
        i++;
    }

    char **frameStrings = backtrace_symbols(&backtrace_frames[0], [addresses count]);

    if(frameStrings != NULL) {
        int x;
        for(x = 0; x < [addresses count]; x++) {
            // Loop over the description of each stack frame.
            // Try to find the symbol name and demangle it if possible

            if(frameStrings[x] == NULL) { break; }

            char * demangled = NULL;
            size_t length;
            int status;

            frame_description = [NSString stringWithUTF8String:frameStrings[x]];
            NSArray * components = [frame_description componentsSeparatedByCharactersInSet:[NSCharacterSet whitespaceCharacterSet]];

            int count = 0;
            for (NSString * component in components)
            {
                if ([component length] == 0)
                    continue;

                count++;
                if (count == 4)
                {
                    symbol = component;
                    demangled = __cxxabiv1::__cxa_demangle([symbol UTF8String], 
                                                           demangled, 
                                                           &length, &status);
                    break;
                }
            }

            if (demangled)
            {
                NSString * demangled_string = [NSString stringWithUTF8String:demangled];
                frame_description = [frame_description stringByReplacingOccurrencesOfString:symbol withString:demangled_string];
                free(demangled);
            }

            trace = [trace stringByAppendingFormat:@"%@\n", frame_description];
        }
        free(frameStrings);
    }
    return trace;
}