How to ignore false positive memory leaks from _CrtDumpMemoryLeaks? - c++

Question

It seems whenever there are static objects, _CrtDumpMemoryLeaks returns a false positive claiming it is leaking memory. I know this is because they do not get destroyed until after the main() (or WinMain) function. But is there any way of avoiding this? I use VS2008.

Answer 1

I found that if you tell it to check memory automatically after the program terminates, it allows all the static objects to be accounted for. I was using log4cxx and boost which do a lot of allocations in static blocks, this fixed my "false positives"...
Add the following line, instead of invoking _CrtDumpMemoryLeaks, somewhere in the beginning of main():
_CrtSetDbgFlag ( _CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF );
For more details on usage and macros, refer to MSDN article:
http://msdn.microsoft.com/en-us/library/5at7yxcs(v=vs.71).aspx

Answer 2

Not a direct solution, but in general I've found it worthwhile to move as much allocation as possible out of static initialization time. It generally leads to headaches (initialization order, de-initialization order etc).
If that proves too difficult you can call _CrtMemCheckpoint (http://msdn.microsoft.com/en-us/library/h3z85t43%28VS.80%29.aspx) at the start of main(), and _CrtMemDumpAllObjectsSince
at the end.

Answer 3

1) You said:
It seems whenever there are static objects, _CrtDumpMemoryLeaks returns a false positive claiming it is leaking memory.
I don't think this is correct. EDIT: Static objects are not created on heap. END EDIT: _CrtDumpMemoryLeaks only covers crt heap memory. Therefore these objects are not supposed to return false positives.
However, it is another thing if static variables are objects which themselves hold some heap memory (if for example they dynamically create member objects with operator new()).
2) Consider using _CRTDBG_LEAK_CHECK_DF in order to activate memory leak check at the end of program execution (this is described here: http://msdn.microsoft.com/en-us/library/d41t22sb(VS.80).aspx). I suppose then memory leak check is done even after termination of static variables.

Answer 4

Old question, but I have an answer. I am able to split the report in false positives and real memory leaks. In my main function, I initialize the memory debugging and generate a real memory leak at the really beginning of my application (never delete pcDynamicHeapStart):
int main()
{
_CrtSetDbgFlag( _CRTDBG_ALLOC_MEM_DF | _CRTDBG_LEAK_CHECK_DF );
char* pcDynamicHeapStart = new char[ 17u ];
strcpy_s( pcDynamicHeapStart, 17u, "DynamicHeapStart" );
...
After my application is finished, the report contains
Detected memory leaks!
Dumping objects ->
{15554} normal block at 0x00000000009CB7C0, 80 bytes long.
Data: < > DD DD DD DD DD DD DD DD DD DD DD DD DD DD DD DD
{14006} normal block at 0x00000000009CB360, 17 bytes long.
Data: <DynamicHeapStart> 44 79 6E 61 6D 69 63 48 65 61 70 53 74 61 72 74
{13998} normal block at 0x00000000009BF4B0, 32 bytes long.
Data: < ^ > E0 5E 9B 00 00 00 00 00 F0 7F 9C 00 00 00 00 00
{13997} normal block at 0x00000000009CA4B0, 8 bytes long.
Data: < > 14 00 00 00 00 00 00 00
{13982} normal block at 0x00000000009CB7C0, 16 bytes long.
Data: < # > D0 DD D6 40 01 00 00 00 90 08 9C 00 00 00 00 00
...
Object dump complete.
Now look at line "Data: <DynamicHeapStart> 44 79 6E 61 6D 69 63 48 65 61 70 53 74 61 72 74".
All reportet leaks below are false positives, all above are real leaks.
False positives don't mean there is no leak (it could be a static linked library which allocates heap at startup and never frees it), but you cannot eliminate the leak and that's no problem at all.
Since I invented this approach, I never had leaking applications any more.
I provide this here and hope this helps other developers to get stable applications.

Answer 5

Can you take a snapshot of the currently allocated objects every time you want a list? If so, you could remove the initially allocated objects from the list when you are looking for leaks that occur in operation. In the past, I have used this to find incremental leaks.
Another solution might be to sort the leaks and only consider duplicates for the same line of code. This should rule out static variable leaks.
Jacob

Answer 6

Ach. If you are sure that _CrtDumpMemoryLeaks() is lying, then you are probably correct. Most alleged memory leaks that I see are down to incorect calls to _CrtDumpMemoryLeaks(). I agree entirely with the following; _CrtDumpMemoryLeaks() dumps all open handles. But your program probably already has open handles, so be sure to call _CrtDumpMemoryLeaks() only when all handles have been released. See http://www.scottleckie.com/2010/08/_crtdumpmemoryleaks-and-related-fun/ for more info.

Answer 7

I can recommend Visual Leak Detector (it's free) rather than using the stuff built into VS. My problem was using _CrtDumpMemoryLeaks with an open source library that created 990 lines of output, all false positives so far as I can tell, as well as some things coming from boost. VLD ignored these and correctly reported some leaks I added for testing, including in a native DLL called from C#.