Objective-C strange EXC_BAD_ACCESS

Question

My code is crashing with an EXC_BAD_ACCESS error that I don't know how to debug.

This is the code:

NSUInteger lineCount = self.lineBeginnings.count;
NSUInteger lineBeginnings[lineCount];
[self.lineBeginnings getIndexes:lineBeginnings maxCount:lineCount inIndexRange:nil];

It crashes on the last line, with EXC_BAD_ACCESS (code=2, address=0x...).

Notice two lines above, it's able to read self.lineBeginnings perfectly, but in the debugger I get:

(lldb) p [self lineBeginnings]
error: Trying to put the stack in unreadable memory at: 0x7fff5d15e310.
(lldb) p _lineBeginnings
(NSMutableIndexSet *) $1 = 0x0000610000059b90
(lldb) po _lineBeginnings
[no Objective-C description available]

Also lineBeginnings doesn't show up properly in the GUI scope browser (all other variables do) and trying to "View Memory of lineBeginnings" gives a completely empty memory view.

The lineBeginnings variable is stored as a strong @property, it's a mutable index set created in the app delegate's init method and never removed while the app is running. There is a background operation queue that writes to it, but it switches to the main thread for all modifications using dispatch_sync(dispatch_get_main_queue()).

I'm not sure how to debug this further? It's hard to reproduce, I have to resize the window for up to a minute (which causes the lineBeginnings variable to be re-created on a background queue, a process that takes ~5 minutes when given 180MB of data), in order to make this crash occur.

It looks like a buffer overrun or something to me? How do I track it down?

The source code for the file is here: https://gist.github.com/abhibeckert/7128740 (the crash is on line 254).

Answer 1

In 180 MB there are probably millions of line beginnings? So you are allocating an array of millions of eight byte words on the stack. A thread's stack usually is not that large.

You should allocate the array on the heap, using malloc.

Your problem is called a stack overflow. Sounds familiar?