Getting info about bad memory address in LLDB
I am trying to debug an EXC_BAD_ACCESS in my iPhone app. It is crashing on a method call and on the line of the method is EXC_BAD_ACCESS (code=1, address = xxx).
Before, I would have just used gdb info malloc-history <xxx> to start debugging, but I am having trouble finding a parallel command in LLDB.
I saw this thread that said to use Instruments, but when I do I still get the crash but I can't figure out how to tell exactly where the app is crashing from in Instruments.
I just need to figure out where this piece of memory that is crashing was pointing to. What is the best way to do this either using LLDB or Instruments?
Answer
You can see the malloc stack if you debug using instruments.
I encountered the same problem as you and similarly wanted to know how to get the malloc history when using lldb. Sadly I didn't find a nifty command like malloc-history found in gdb. To be honest I just switched my debugger over, but I found that annoying since I felt I shouldn't have to do that.
To find the malloc history using instruments:
- Profile your project
- Select Zombies from the list of instruments
- Make your app trigger the problem
- At this point you should be presented with the address that was already deallocated and you can explore it.
It should be a simple matter of viewing the malloc history at this point. I blacked out portions that had class / project names specific to the work I'm doing, but I think the essence and usefulness of how to go about getting this information is present.
A Last Word
The problem I ran into yielded a message like:
*** -[someClass retain]: message sent to deallocated instance 0x48081fb0 someProject(84051,0xacd902c0) malloc: recording malloc stacks to disk using standard recorder
I was really puzzled where this retain was coming from since the code it was breaking on didn't have one (not in the getter or setter of the line it was on). It turns out that I was not calling removeObserver:forKeyPath: when a certain object was dealloc'ed. Later in execution KVO occurred do to a setter on a line and that blew up the program since KVO was trying to notify an object that was already released.