How to find the cause of a malloc “double free” error?

by

When an object is “double-freed”, the most common cause is that you’re (unnecessarily) releasing an autoreleased object, and it is later autoreleased when the containing autorelease pool is emptied.

I’ve found that the best way to track down the extra release is to use the NSZombieEnabled environment variable for the affected executable in Xcode. For a quick rundown of how to use it, check out this CocoaDev wiki page. (In addition to this page, Apple has documented some incredibly obscure yet useful tips for debugging code in Xcode, some of which have saved my bacon more than a few times. I suggest checking out this Technical Note on developer.apple.com — link jumps to the section on Cocoa’s Foundation framework).

Edit: You can often track the offending object down within the Xcode debugger, but it’s often much easier if you use Instruments to assist you. From Xcode, choose Run → Start With Performance Tool → Object Allocations and you should be able to trace the offending object back to where it was created. (This will work best if you’re enabled zombies as discussed above.) Note: Snow Leopard adds a Zombies tool to Instruments, accessible from the Run menu as well. Might be worth the $29 alone! 😉

There is also a related SO question here.

More Related Contents:

  1. When should I release objects in -(void)viewDidUnload rather than in -dealloc?
  2. Memory Management in Objective-C [duplicate]
  3. iOS Low Memory Crash, but very low memory usage
  4. Why is autorelease especially dangerous/expensive for iPhone applications?
  5. Objective c, Memory management of instance members
  6. iPhone development: pointer being freed was not allocated
  7. Problem dealloc’ing memory used by UIImageViews with fairly large image in an UIScrollView
  8. Custom colors in UITabBar
  9. How can I get a precise time, for example in milliseconds in Objective-C?
  10. When do you make an underscore in front of an instance variable? [duplicate]
  11. program access to iPhone volume buttons
  12. Change iOS app’s language on the fly
  13. Autorotate in iOS 6 has strange behaviour
  14. How to resize the image programmatically in objective-c in iphone
  15. iPhone: Convert date string to a relative time stamp
  16. iPhone – What are reuseIdentifiers (UITableViewCell)?
  17. How many times do I release an allocated or retained object?
  18. Removing a view controller from UIPageViewController
  19. How to rotate an image 90 degrees on iOS?
  20. Calling a method on an uninitialized object (null pointer)
  21. Method Swizzling in iOS 5?
  22. Changing the background color of a UIAlertView?
  23. Error : The service is invalid
  24. Iphone UITextField only integer
  25. How to handle duplicate symbol error from 3rd party libraries?
  26. Accessing UITextField in a custom UITableViewCell
  27. How to tint UIBarButtonItem background color? [duplicate]
  28. iOS – Scale and crop CMSampleBufferRef/CVImageBufferRef
  29. UISwitch in a UITableView cell
  30. How to set font size to fill UILabel height?

Leave a Comment