iOS - No stack trace when calling dispatch_async

Question

I've created some example code to demonstrate my problem.

- (void) test {

    void (^handler)(void)  = ^ {

        NSArray *test = [NSArray array];
        [test objectAtIndex: 5];
    };

    handler = [handler copy];

    dispatch_async(dispatch_get_main_queue(), handler);
}

When I call the test method I don't get a stack trace. The debugger just stops at main.m and has this line highlighted

int retVal = UIApplicationMain(argc, argv, nil, NSStringFromClass([FantasyUniversalAppDelegate class]));

If I remove the dispatch_async and just call handler(); then I do get a stack trace. Can some one explain why this is and is there a way to get a more descriptive stack trace?

Right now I am using Flurry to show me crashes but the crashes it is showing me isn't very useful because I get a very non-descriptive stack trace (which is symbolicated). It looks like this:

Thread: Unknown Name
0     libsystem_kernel.dylib                0x3a224eb4 mach_msg_trap + 20
1     CoreFoundation                        0x32067045 __CFRunLoopServiceMachPort + 129
2     CoreFoundation                        0x32065d5f __CFRunLoopRun + 815
3     CoreFoundation                        0x31fd8ebd CFRunLoopRunSpecific + 357
4     CoreFoundation                        0x31fd8d49 CFRunLoopRunInMode + 105
5     GraphicsServices                      0x35b9c2eb GSEventRunModal + 75
6     UIKit                                 0x33eee301 UIApplicationMain + 1121
7     IOSTestApp                        0x00025d7b main (main.m:14)

Answer 1

You could wrap your code in a

@try {
    NSArray *test = [NSArray array];
    [test objectAtIndex: 5];
}
@catch {
    // Get the stack track for the current thread
    NSArray *stacktrace = [NSThread callStackSymbols];
    // Do something with the symbols
}

all within the block that you are dispatching. Though I'm sure there is a much nicer way!

Answer 2

iOS async stack trace record https://github.com/tiantianbobo/XBAsyncStackTrace

Suppose codes like this when compiled in release version:

- (void)callCrashFunc {
id object = (__bridge id)(void*)1;
[object class];
//    NSLog(@"remove this line will cause tail call optimization");
}
- (void)testDispatchAsyncCrash {
    dispatch_async(dispatch_get_global_queue(0, 0), ^{
        [self callCrashFunc];
    });
}

normal stack trace methods only get you this:

frame #0: 0x000000010186fd85 libobjc.A.dylib`objc_msgSend + 5
    frame #1: 0x0000000104166595 libdispatch.dylib`_dispatch_call_block_and_release + 12
    frame #2: 0x0000000104167602 libdispatch.dylib`_dispatch_client_callout + 8
    frame #3: 0x000000010416a064 libdispatch.dylib`_dispatch_queue_override_invoke + 1028
    frame #4: 0x000000010417800a libdispatch.dylib`_dispatch_root_queue_drain + 351
    frame #5: 0x00000001041789af libdispatch.dylib`_dispatch_worker_thread2 + 130
    frame #6: 0x0000000104553169 libsystem_pthread.dylib`_pthread_wqthread + 1387
    frame #7: 0x0000000104552be9 libsystem_pthread.dylib`start_wqthread + 13

But you still don't know where actual code is.

However, XBAsyncStackTrace will give you:

0   XBAsyncStackTraceExample            0x000000010c89d75c blockRecordAsyncTrace + 76
1   XBAsyncStackTraceExample            0x000000010c89d302 wrap_dispatch_async + 98
2   XBAsyncStackTraceExample            0x000000010c89c02c -[ViewController testDispatchAsyncCrash] + 92
3   XBAsyncStackTraceExample            0x000000010c89be3d -[ViewController viewDidLoad] + 269
4   UIKitCore                           0x0000000110ae44e1 -[UIViewController loadViewIfRequired] + 1186
5   UIKitCore                           0x0000000110ae4940 -[UIViewController view] + 27

Bingo!