Re: Why does MFC call _CrtDumpMemoryLeaks ?!?

"Doug Harrison [MVP]" <>
Fri, 11 Jul 2008 11:34:08 -0500
On Fri, 11 Jul 2008 15:28:23 +0200, "Martin T." <> wrote:

(I think this applies to mfc & C++ & debugging, hence the 3 NG)

OK, maybe this'll be some kind of rant, but I really don't get it and it
would really be nice to know what the rationale behind the "built-in"
memory leak detection of MFC is, when any user could provide it on his
own without false memory leaks ...

The Run-Time Library provides for Debug Routines that (among other
things) allow reporting of memory-leaks.
It is possible to enable *automatic* reporting for the CRT by calling
the function _CrtSetDbgFlag(...) with the _CRTDBG_LEAK_CHECK_DF flag.
If you choose to enable this *automatic* check in your C++ program, the
CRT will happily (and most correctly) report all left-over memory via
_CrtDumpMemoryLeaks *after* basically all non-system DLLs have been

Enter MFC:
For reasons unknown to me (but probably historical or some such $%&$)
/if/ your Application links against the MFC DLL /then/ within the d'tor
of a class called _AFX_DEBUG_STATE (atlmfc/src/mfc/dumpinit.cpp, ln 123)
MFC will call _CrtDumpMemoryLeaks and afterwards disable the reporting
on process-end:
    int nOldState = _CrtSetDbgFlag(_CRTDBG_REPORT_FLAG);
    _CrtSetDbgFlag(nOldState & ~_CRTDBG_LEAK_CHECK_DF);

    ASSERT(_CrtSetReportHook2(_CRT_RPTHOOK_REMOVE,_AfxCrtReportHook) != -1);
#endif // _AFX_NO_DEBUG_CRT

Not only is this completely unnecessary, as you could (if you so wished)
tell the CRT to dump it on process end - oh no! It is quite horrible if
you use a 3rd party C++ DLL that happens to hold memory in global static
objects that will only be destroyed when this DLL is unloaded and does
*not* link against MFC.
That is because in all my test cases any user-supplied DLL (project
dependency, via Linker-Input:lib) is always unloaded *after* the MFC-DLL
(mfc80ud.dll in my case).
In this scenario the stupid call of _CrtDumpMemoryLeaks from the MFC
cleanup code will report all this memory as leaks, even though it *will*
be cleaned up --- it just happens that the code had the cheek to not use

* You can't work around it, as you cannot change DLL (static)
load/unload order.
* You can't work around it, as you cannot use a C++ DLL with
* You can't work around it, as you cannot tell MFC not to report this
false memleaks and use the CRT mechanism instead.
* You can't work around it, as your main application needs to use MFC.
* You can't work around it, as the 3rd party DLL cannot, in general, be
made to link against MFC (thereby forcing a later unload of mfc80ud.dll)
* You can't even ignore the MFC dump and additionally use the CRT dump
as MFC will switch it off after it dumped!


So ... I'm stuck with a program that dumps 2000 memory leaks at
MFC-cleanup that are false and will obscure any real (and possibly
problematic) memleaks I would like to find ...

It would be interesting to hear from someone who knows a rational behind
the MFC code or has some general thoughts how to get around it or to
hear why I'm completely mistaken :)

The rationale is that it was a mistake. (The real question is, "Why won't
they fix it?") For a workaround for those non-MFC DLLs you mentioned, see
if these threads help:

Though I never tried it, the second one should help with those DLLs you
can't modify.

Doug Harrison
Visual C++ MVP

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