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Debugging memory corruption? (C++)
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Debugging memory corruption? (C++)


  • Subject: Debugging memory corruption? (C++)
  • From: Jean-Denis Muys <email@hidden>
  • Date: Fri, 30 Oct 2009 16:24:11 +0100

Hi,

My code is corrupting memory and I have a hard time finding the bug. Perhaps somebody can point me to some tool or idea that has escaped me so far.

- Context: my code is a plugin written in C++ for another application (4D). Yesterday everything was OK, and then I re-factored some code to migrate it to a more light weight class hierarchy, in order to have a better division of work in prevision of another set of features to implement. Now my code crashes in one of the host application callbacks.

- tools that I cannot use:
Unfortunately the host app crashes at launch time under Instruments or MallocDebug. So I can't use them. I probably can't use the XCode "Enable Guard Malloc" command either. I tried, but after waiting for about an hour for the host app to launch, I stopped. It was either crashed or so tremendously slowed down that I couldn't do anything useful.

- tools I can and have used:
• leaks works fine and I use it daily, but is not useful for this kind of issues
• I enabled all the malloc() debugging environment variables, and indeed, malloc reports interesting things, for example:

4D(4223,0xb03cb000) malloc: *** previous incorrectly set slot=0  count=4 ptr=0x49c2ed0
4D(4223,0xb03cb000) malloc: *** tiny free list incorrect (slot=0) szone_check_all() counter=54572
*** error: check: tiny free list incorrect
*** set a breakpoint in malloc_error_break to debug

or more frequently:

4D(4154,0xb02e9000) malloc: *** error for object 0x4ba1560: incorrect checksum for freed object - object was probably modified after being freed.
*** set a breakpoint in malloc_error_break to debug

Its useful, but when I break there, it too late. My buggy code has misbehaved already, possibly quite a bit ago.

One question comes to mind right away: is there a way to call the malloc heap-checking routines from my program? I could then sprinkle my code where it makes sense with heap checks.

Somewhat related is that I have set MallocCheckHeapEach to 100. This already slows the app down quite a bit. I can probably set to an even smaller value, but I don't think I can set it to 1.

Or if I change it to 1 *at run time* how can I inform malloc that I want the frequency to change? AFAICS, it only reads those environment variables at launch time.

Any suggestion welcome (also if I have missed some debugging tools).

Jean-Denis
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