EXC_BAD_ACCESS (code=2, address=0x16d0f3ff0)

Question

Hi!

I am developing an app (in XCode Version 11.2 and Swift 4.2) in which I fill in a LinkedList and after working with it, removing the elements that compose it produces the Thread 1 error: EXC_BAD_ACCESS (code = 2, address = 0x16d0f3ff0). The error occurs even without working with the items in the list, simply by adding them and trying to eliminate them the error already occurs. The tests I am doing with an iPhone with IOS version 11.4.1

The method that eliminates the elements of the LinkedList is removeAll.

The error occurs in the first line:

public func removeAll() {
  self.head = nil    //Thread 1: EXC_BAD_ACCESS (code=2, address=0x16d0f3ff0)
  self.tail = nil
  self.count = 0
} // removeAll

the definition of the variables 'head', 'tail', 'count':

  private var head: Node<T>?
  private var tail: Node<T>?
  public private(set) var count: Int = 0

And the type Node<T>:

public class Node<T> {
  var value: T
  var next: Node<T>?
  weak var previous: Node<T>?
  init(value: T) {
  self.value = value
  } // init
} // Node

With the debugger I see that the error occurs when assigning the value nil to self.head (in removeAll()). Just before self.head has correct values, it becomes nil and the error is skipped before reaching the next instruction.

In the Debug Navigator, in the stackTrace the last 2 functions where the error is seen:

libobjc.A.dylib`_object_remove_assocations:
  0x180d11eec <+0>: sub sp, sp, #0x70 ; =0x70 
-> 0x180d11ef0 <+4>: stp x26, x25, [sp, #0x20] //Thread 1 error: EXC_BAD_ACCESS (code = 2, address = 0x16d0f3ff0)
  0x180d11ef4 <+8>: stp x24, x23, [sp, #0x30]
libswiftCore.dylib`_swift_release_:
  0x104e18d1c <+180>: bl 0x104e1a37c ; bool swift::RefCounts<swift::SideTableRefCountBits>::doDecrement<(swift::PerformDeinit)1>(unsigned int)
-> 0x104e18d20 <+184>: ldp x29, x30, [sp], #0x10. //Thread 1 error: EXC_BAD_ACCESS (code = 2, address = 0x16d0f3ff0)
  0x104e18d24 <+188>: ret

Does someone have idea how to resolve it?

Thanks!

Swift

20k

Posted by

arekusu

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Answer 1

Something like this, removing the last element in the linked list:

    public func removeTail() {
        guard let tail = tail else {return}
        if let prev = tail.previous {
            prev.next = nil
            self.tail = prev
        } else {
            self.head = nil
            self.tail = nil
        }
        count -= 1
    }

Posted by

OOPer

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Answer 2

How long is the linked list when this error occures?

In my experience, too many linked objects may cause such an issue under ARC.

Posted by

OOPer

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Answer 3

In this case, 4389 elements. How many elements approximately can produce this error?

Posted by

arekusu

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Answer 4

That number is far less than I experienced, but depends on many things.

With a simplified project I created, 50000 is enough to reproduce the same issue.

Please try this:

    public func removeAll() {
        while head != nil {
            removeTail()
        }
    }

(If you do not have `removeTail()` yet, please define it.)

Posted by

OOPer

This approach solved the crash with my cpp implementation

—
BrandonMFong

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Answer 5

I do not understand. That will make it stay in an infinite loop, right? What should removeTail do?

Posted by

arekusu

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Answer 6

Something like this, removing the last element in the linked list:

    public func removeTail() {
        guard let tail = tail else {return}
        if let prev = tail.previous {
            prev.next = nil
            self.tail = prev
        } else {
            self.head = nil
            self.tail = nil
        }
        count -= 1
    }

Posted by

OOPer

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Answer 7

It works perfectly. I was totally blocked with this. Thank you very much!!

Posted by

arekusu

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Answer 8

I investigated right today a similar issue happening in my code.

The error Thread 1: EXC_BAD_ACCESS (code=2, address=0x101744d41) was happening at line 8

    var emptyUsersListView: some View {
        List {
            EmptyView()
        }
        .navigationBarItems(trailing:
            Button(store.state.loginButtonText) {
                self.store.send(.tapOnLogin)
            }
        )
    }

After hours of digging, I narrowed it down to the Store class.

The crashed thread stack trace gave me the hint: the top frame is about the objectWillChangePublisher of Combine ObservableObject.

#0 0x0000000101744d41 in got.$s25ObjectWillChangePublisher7Combine010ObservableA0PTl ()
#1 0x0000000101571bcb in closure #2 in RootContainerView.emptyUsersListView.getter
#2 0x00007fff2c014579 in PrimitiveButtonStyleConfiguration.trigger() ()
#3 0x00007fff2c12bd20 in partial apply for PrimitiveButtonStyleConfiguration.trigger() ()
#4 0x00007fff2c35b67a in closure #1 in PressableGestureCallbacks.dispatch(phase:state:) ()
#5 0x00007fff2c1b7cec in thunk for @escaping @callee_guaranteed () -> () ()
#6 0x00007fff2c316d21 in partial apply for thunk for @escaping @callee_guaranteed () -> () ()
#7 0x00007fff2c317b79 in thunk for @escaping @callee_guaranteed () -> ()partial apply ()

So I had a closer look at the Store class, whose property sections look a bit like this.

    // MARK: - Public properties
    
    public var objectWillChange: AnyPublisher<_Reducer.State, Never> {
        return objectWillChangeSubject.eraseToAnyPublisher()
    }

    public private(set) var state: _Reducer.State {
        willSet { objectWillChangeSubject.send(newValue) }
    }

    // MARK: - Private properties

    private let objectWillChangeSubject = PassthroughSubject<_Reducer.State, Never>()

Do you see what your code and my code have in common? The following line:

   public private(set) var foo: Bar

So today, just without even knowing what I was doing, I changed the access modifiers of foo to

     public var foo: Bar

and that fixed the issue!!! I checked that also this version works.

     public internal(set) var foo: Bar

Now I would love to understand why this fixed the issue.

As much as I would like to know if such a fix would work in your case too.

Is public private(set) the real reason of the crash?

My Store class would be way more robust with the private modifier, but meanwhile, I can live with the less restricted modifier.

Posted by

codedby.pm

I had the same crash but it occurred when accessing the auto-generated initializer for a struct.

I'd do a clean, and then build, and it would crash with an "exc_bad_access" code=2. The stack trace ends with a call to libswiftCore.dylib`swift_retain

I'd then manually code the initializer and it'd be fine. If I commented out the code, without cleaning, it'd still be fine which was odd.

But if you clean it again, and then build it using the auto-generated initializer, it'd crash again.

It seems that the auto-generated code is causing some issues. This also seems to be the case for codedby.pm too as he discussed changing a property from: public private(set) var foo: Bar to: public var foo: Bar removed the crash.

—
dtanmasimo

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