Re: How to find EXC_BAD_ACCESS bug?

FROM : Sean McBride
DATE : Thu Aug 26 04:58:25 2004

Steve Sims (<email_removed>) on Tue, Aug 24, 2004 19:06 said:

>Often I will see a malloc error before my app crashes, reporting an
>incorrect checksum for a freed object.


Have you tried breaking there, breakpoint on malloc_printf()?

>I'm at a bit of a loss right now as to how to track this bug down
>further.  Any suggestions would be most gratefully received.


Well, I once spent a month or so trying to find a bug that sounds like
yours and here are some of the things I'd suggest, in no order:

- Use everything in NSDebug.h, not just NSZombieEnabled
- use all the malloc env vars, especially set MallocCheckHeapEach to 1
- for C++ new/delete try CodeWarrior's DebugNew (NB its not thread safe)
- try a different IDE/compiler to rule out bugs there
- try different Macs, RAM, OS versions to rule out bugs there
- fix all memory leaks (see MallocDebug, 'leaks' command line tool)
- try malloc_history
- try Apple's guard malloc, or any of the other replacement malloc
implementations.
- use debug variants of Apple's frameworks (easiest by setting
DYLD_IMAGE_SUFFIX env var to _debug)
- rip out code and use process of elimination :(


(My particular bug was nasty, I couldn't use all the debugging facilities
fully, like MallocCheckHeapEach=1, because it was code that interfaced
with hardware, and if I slowed it down too much nothing worked at all.
It also took about 5 hours to repro the bug! :(  In the end I had to rip
out code until it went away... in the end, it was a QuickTime bug, fixed
in 10.3.  One month down the drain.  But I learned a lot about debugging.
hehe)
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