Understanding Zombies in XCode for Debugging EXC_BAD_ACCESS Issues

As a developer, we’ve all encountered the frustrating EXC_BAD_ACCESS error at some point. It’s a signal that occurs when an application attempts to access memory that it doesn’t have permission to access. While this can be challenging to debug, there is a powerful tool available in XCode known as “Zombies” that can help us identify the root cause of the issue.

In this article, we’ll delve into the world of Zombies and explore how they can aid in debugging EXC_BAD_ACCESS errors. We’ll also provide step-by-step instructions on how to enable Zombies in your project and interpret the debug information they provide.

What are Zombies?

Zombies is a debugging technique that allows us to simulate the behavior of an object even after it has been deallocated from memory. In other words, when an object is deallocated, its memory is released back to the heap, but Zombies allows us to treat it as if it still exists in memory.

By enabling Zombies, we can simulate the behavior of objects that have been deallocated, which can help us identify the cause of EXC_BAD_ACCESS errors. This technique relies on the fact that many Objective-C classes implement a custom dealloc method, which is called when an object is deallocated from memory.

Enabling Zombies in XCode

To enable Zombies in your project, follow these steps:

1. Open your XCode project and navigate to the “Product” menu.
2. Click on “Scheme” and select “Edit Scheme”.
3. In the scheme editor, click on the “Arguments” tab.
4. Scroll down to the “Variables to be set in the environment” section.
5. Set the value of NSZombieEnabled to "YES".

This will enable Zombies for your project and allow you to debug EXC_BAD_ACCESS errors using this technique.

Interpreting Zombie Debug Information

When Zombies are enabled, XCode will simulate the behavior of objects that have been deallocated from memory. This information is available in the console output, where we can see messages indicating which objects were accessed after they had been deallocated.

Here’s an example of what we might see in the console output:

This GDB was configured as "x86_64-apple-darwin".Setting environment variable "NSZombieEnabled" to null value.
Program received signal:  “EXC_BAD_ACCESS”.

In this case, we can see that a program received an EXC_BAD_ACCESS error. However, the interesting part is what comes next:

*** __assert2_get_handler + 0x7b9 (0x100003a60) at -[NSObject _objectDidRelease] + 0x1204 (0x100000f50)
*** __assert2_get_handler + 0x7c1 (0x100003a81) at -[NSObject _didMemoryPressureNotification:] + 0x1103 (0x100001f40)
*** __assert2_get_handler + 0x6a4 (0x1000038d0) at -[NSApplication run] + 0x10e1 (0x1000028b1)

This output indicates that the program received an EXC_BAD_ACCESS error, but it also provides information about which object was accessed after it had been deallocated. In this case, the object _objectDidRelease was accessed.

How Zombies Help with Debugging EXC_BAD_ACCESS Issues

So, how can Zombies help us debug EXC_BAD_ACCESS issues? By simulating the behavior of objects that have been deallocated from memory, Zombies provides valuable information about which objects were accessed after they had been released.

This can be particularly helpful in identifying the root cause of EXC_BAD_ACCESS errors. For example, if we see an object being accessed after it has been deallocated, it may indicate a problem with our code that is causing the object to be released prematurely.

Here’s an example:

*** __assert2_get_handler + 0x7b9 (0x100003a60) at -[MyObject dealloc] + 0x1204 (0x100000f50)
*** __assert2_get_handler + 0x7c1 (0x100003a81) at -[MyClass _didMemoryPressureNotification:] + 0x1103 (0x100001f40)

In this case, we can see that the object MyObject was accessed after it had been deallocated from memory. This may indicate a problem with our code that is causing the object to be released prematurely.

Conclusion

In conclusion, Zombies are a powerful tool for debugging EXC_BAD_ACCESS issues in XCode. By simulating the behavior of objects that have been deallocated from memory, Zombies provide valuable information about which objects were accessed after they had been released.

By enabling Zombies and interpreting the debug information they provide, we can identify the root cause of EXC_BAD_ACCESS errors and fix our code accordingly. Remember to set NSZombieEnabled to "YES" in your project’s scheme settings, and don’t be afraid to experiment with different debugging techniques to find the solution to your problem.

Additional Tips and Tricks

Here are some additional tips and tricks for working with Zombies:

• Use Zombies sparingly: While Zombies can be a powerful tool for debugging EXC_BAD_ACCESS issues, they should only be used when necessary. Enabling Zombies can slow down your application’s performance, so use them judiciously.
• Understand the _objectDidRelease message: The _objectDidRelease message is sent to objects that have been deallocated from memory. Understanding this message and how it relates to our code can help us identify problems with object retention.
• Experiment with different debugging techniques: While Zombies are a powerful tool, they may not always provide the answers we’re looking for. Experimenting with different debugging techniques, such as using the debugger or analyzing memory dumps, can sometimes provide more insight into our problem.

Example Code

Here’s an example of how to use Zombies in our code:

#import <Foundation/Foundation.h>

@interface MyObject : NSObject {
    @private
    NSValue *_releaseDate;
}

- (instancetype)init {
    self = [super init];
    if (self) {
        _releaseDate = [NSValue valueWithTimeIntervalSinceNow:10.0];
    }
    return self;
}

- (void)dealloc {
    // Simulate object release
    [_releaseDate getValue:&_releaseDate];
}

@end

@implementation MyObject

- (void)testZombie {
    // Create an instance of MyObject and access it after it has been deallocated from memory
    MyObject* obj = [[MyObject alloc] init];
    [obj testZombie]; // This will cause the object to be accessed after it has been released
}

@end

By enabling Zombies and analyzing the debug information they provide, we can identify problems with our code that are causing EXC_BAD_ACCESS errors.

Last modified on 2024-04-25