[c++] 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.

In addition to looking for tools, consider looking for a likely culprit. Is there any component you're using, perhaps not written by you, which may not have been designed and tested to run in a multithreaded environment? Or simply one which you do not know has run in such an environment.

The last time it happened to me, it was a native package which had been successfully used from batch jobs for years. But it was the first time at this company that it had been used from a .NET web service (which is multithreaded). That was it - they had lied about the code being thread safe.

You may also want to check to see whether you're linking against the dynamic or static C runtime library. If your DLL files are linking against the static C runtime library, then the DLL files have separate heaps.

Hence, if you were to create an object in one DLL and try to free it in another DLL, you would get the same message you're seeing above. This problem is referenced in another Stack Overflow question, Freeing memory allocated in a different DLL.

I'd like to add my experience. In the last few days, I solved an instance of this error in my application. In my particular case, the errors in the code were:

• Removing elements from an STL collection while iterating over it (I believe there are debug flags in Visual Studio to catch these things; I caught it during code review)
• This one is more complex, I'll divide it in steps:
  • From a native C++ thread, call back into managed code
  • In managed land, call Control.Invoke and dispose a managed object which wraps the native object to which the callback belongs.
  • Since the object is still alive inside the native thread (it will remain blocked in the callback call until Control.Invoke ends). I should clarify that I use boost::thread, so I use a member function as the thread function.
  • Solution: Use Control.BeginInvoke (my GUI is made with Winforms) instead so that the native thread can end before the object is destroyed (the callback's purpose is precisely notifying that the thread ended and the object can be destroyed).

I had a similar problem - and it popped up quite randomly. Perhaps something was corrupt in the build files, but I ended up fixing it by cleaning the project first then rebuilding.

So in addition to the other responses given:

What sort of things can cause these errors? Something corrupt in the build file.

How do I debug them? Cleaning the project and rebuilding. If it's fixed, this was likely the problem.

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.

One quick tip, that I got from Detecting access to freed memory is this:

If you want to locate the error quickly, without checking every statement that accesses the memory block, you can set the memory pointer to an invalid value after freeing the block:

#ifdef _DEBUG // detect the access to freed memory
#undef free
#define free(p) _free_dbg(p, _NORMAL_BLOCK); *(int*)&p = 0x666;
#endif

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

If these errors occur randomly, there is high probability that you encountered data-races. Please, check: do you modify shared memory pointers from different threads? Intel Thread Checker may help to detect such issues in multithreaded program.

The best tool I found useful and worked every time is code review (with good code reviewers).

Other than code review, I'd first try Page Heap. Page Heap takes a few seconds to set up and with luck it might pinpoint your problem.

If no luck with Page Heap, download Debugging Tools for Windows from Microsoft and learn to use the WinDbg. Sorry couldn't give you more specific help, but debuging multi-threaded heap corruption is more an art than science. Google for "WinDbg heap corruption" and you should find many articles on the subject.

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.

You can use VC CRT Heap-Check macros for _CrtSetDbgFlag: _CRTDBG_CHECK_ALWAYS_DF or _CRTDBG_CHECK_EVERY_16_DF.._CRTDBG_CHECK_EVERY_1024_DF.

I have also faced this issue. In my case, I allocated for x size memory and appended the data for x+n size. So, when freeing it shown heap overflow. Just make sure your allocated memory sufficient and check for how many bytes added in the memory.

What type of allocation functions are you using? I recently hit a similar error using the Heap* style allocation functions.

It turned out that I was mistakenly creating the heap with the HEAP_NO_SERIALIZE option. This essentially makes the Heap functions run without thread safety. It's a performance improvement if used properly but shouldn't ever be used if you are using HeapAlloc in a multi-threaded program [1]. I only mention this because your post mentions you have a multi-threaded app. If you are using HEAP_NO_SERIALIZE anywhere, delete that and it will likely fix your problem.

[1] There are certain situations where this is legal, but it requires you to serialize calls to Heap* and is typically not the case for multi-threaded programs.

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 );