How to debug heap corruption errors?

I am debugging a (native) multi-threaded C++ application under Visual Studio 2008. On seemingly random occasions, I get a "Windows has triggered a break point..." error with a note that this might be due to a corruption in the heap. These errors won't always crash the application right away, although it is likely to crash short after.

The big problem with these errors is that they pop up only after the corruption has actually taken place, which makes them very hard to track and debug, especially on a multi-threaded application.

• What sort of things can cause these errors?

• How do I debug them?

Tips, tools, methods, enlightments... are welcome.

Application Verifier combined with Debugging Tools for Windows is an amazing setup. You can get both as a part of the Windows Driver Kit or the lighter Windows SDK. (Found out about Application Verifier when researching an earlier question about a heap corruption issue.) I've used BoundsChecker and Insure++ (mentioned in other answers) in the past too, although I was surprised how much functionality was in Application Verifier.

Electric Fence (aka "efence"), dmalloc, valgrind, and so forth are all worth mentioning, but most of these are much easier to get running under *nix than Windows. Valgrind is ridiculously flexible: I've debugged large server software with many heap issues using it.

When all else fails, you can provide your own global operator new/delete and malloc/calloc/realloc overloads -- how to do so will vary a bit depending on compiler and platform -- and this will be a bit of an investment -- but it may pay off over the long run. The desirable feature list should look familiar from dmalloc and electricfence, and the surprisingly excellent book Writing Solid Code:

• sentry values: allow a little more space before and after each alloc, respecting maximum alignment requirement; fill with magic numbers (helps catch buffer overflows and underflows, and the occasional "wild" pointer)
• alloc fill: fill new allocations with a magic non-0 value -- Visual C++ will already do this for you in Debug builds (helps catch use of uninitialized vars)
• free fill: fill in freed memory with a magic non-0 value, designed to trigger a segfault if it's dereferenced in most cases (helps catch dangling pointers)
• delayed free: don't return freed memory to the heap for a while, keep it free filled but not available (helps catch more dangling pointers, catches proximate double-frees)
• tracking: being able to record where an allocation was made can sometimes be useful

Note that in our local homebrew system (for an embedded target) we keep the tracking separate from most of the other stuff, because the run-time overhead is much higher.

If you're interested in more reasons to overload these allocation functions/operators, take a look at my answer to "Any reason to overload global operator new and delete?"; shameless self-promotion aside, it lists other techniques that are helpful in tracking heap corruption errors, as well as other applicable tools.

Because I keep finding my own answer here when searching for alloc/free/fence values MS uses, here's another answer that covers Microsoft dbgheap fill values.

You can detect a lot of heap corruption problems by enabling Page Heap for your application . To do this you need to use gflags.exe that comes as a part of Debugging Tools For Windows

Run Gflags.exe and in the Image file options for your executable, check "Enable Page Heap" option.

Now restart your exe and attach to a debugger. With Page Heap enabled, the application will break into debugger whenever any heap corruption occurs.

To really slow things down and perform a lot of runtime checking, try adding the following at the top of your main() or equivalent in Microsoft Visual Studio C++

_CrtSetDbgFlag(_CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF | _CRTDBG_CHECK_ALWAYS_DF );

A very relevant article is Debugging Heap corruption with Application Verifier and Debugdiag.

What sort of things can cause these errors?

Doing naughty things with memory, e.g. writing after the end of a buffer, or writing to a buffer after it's been freed back to the heap.

How do I debug them?

Use an instrument which adds automated bounds-checking to your executable: i.e. valgrind on Unix, or a tool like BoundsChecker (Wikipedia suggests also Purify and Insure++) on Windows.

Beware that these will slow your application, so they may be unusable if yours is a soft-real-time application.

Another possible debugging aid/tool might be MicroQuill's HeapAgent.