Re: How to catch this error Access Violation

"Ivan Brugiolo [MSFT]" <>
Tue, 27 Feb 2007 18:11:58 -0800
I'm glad that moving the code in FinalRelease fixed the problem.
Your scenario is exactly the scenario why that FinalRelease method
was exposed to the user of the ATL library.

I don't know much about VB, but, I can immagine
that VB is creating an interceptor for every interface pointer
it has knowledge of, and, the VB code is trying to call
CoFreeUnusedLibraries() after it has released the
intercepted interface poitner, in order to flush the
COM cache of in-proc components.

The OS does exactly what it's supposed to do.
When CoFreeUnusedLibraries is called,
any DLL whose DllCanUnloadNow returns S_OK
is elegibile to be unloaded.
That's why you should decrement the module refcont
at the very-very-very last moment.
OLE32 knows that your destructor can still be executing,
and, it employes a delayed-unloading scheme.
Only CoFreeUnusedLibrariesEx(dwUnloadDelay = 0)
causes the immediate unload, and, this is not the default
behavior, and, it's done to compensate for the problem
you have been victim.

As a final consideration, I have seen your problem
quite a few times, and, it's normally exposed by a multithreaded
application with many apartments that comes and goes,
since the Apartment cleanup code normally calls CoFreeUnusedLibraries.

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Use of any included script samples are subject to the terms specified at

<> wrote in message

Here is what I did, I change my code as following

STDAPI DllCanUnloadNow(void)
wchar_t szMag[2000];
long x=_Module.GetLockCount() ;
wsprintf(szMag,L"_Module.GetLockCount() is %ld ",x);
return (_Module.GetLockCount()==0) ? S_OK : S_FALSE;

The output is Module.GetLockCount() is 0 before the AV

There are two fixes
1) As you suggested, move the clean up code to the FinalRelease fixed
the Av
2) If I change the DllCanUnloadNow to always retrun S_FALS also fix
the AV

What I do not understand is how could the OS unload a dll module while
it is in the process of executing its destructor code??? As you could
see from the following trace, looks like something in MSVBVM60!
CCreDestroyCtl unloaded the AppContainer.dll. but why and how?

Here is the Trace in the in the Rlease()
ULONG l = InternalRelease();
if (l == 0)
delete this;
return l;

At the point of
delete this;

I have the following Trace:

ChildEBP RetAddr
0012f448 79e8dbde AppContainer!ATL::CComObject<
CPropertyContaine>::Release+0x2c [c:\program files\microsoft visual
studio\vc98\atl\include\atlcom.h @ 2419]
0012f49c 79e8db4a mscorwks!ReleaseTransitionHelper+0x5f
0012f4e4 79e8dac5 mscorwks!SafeReleaseHelper+0x89
0012f518 79f27983 mscorwks!SafeRelease+0x2f
0012f530 79f2792e mscorwks!RCW::ReleaseAllInterfaces+0x49
0012f560 79f279dc mscorwks!RCW::ReleaseAllInterfacesCallBack+0xbd
0012f590 79f279b0 mscorwks!RCW::Cleanup+0x22
0012f598 79f27997 mscorwks!RCWCleanupList::ReleaseRCWListRaw+0x14
0012f5c8 79f277e5 mscorwks!RCWCleanupList::ReleaseRCWListInCorrectCtx
0012f5d8 77525fbe mscorwks!CtxEntry::EnterContextCallback+0x94
0012f5f4 77e7a19c ole32!CRemoteUnknown::DoCallback+0x7a
0012f610 77ef321a RPCRT4!Invoke+0x30
0012fa14 77ef3bf3 RPCRT4!NdrStubCall2+0x297
0012fa6c 77600c31 RPCRT4!CStdStubBuffer_Invoke+0xc6
0012faac 77600bdb ole32!SyncStubInvoke+0x33
0012faf4 7750f237 ole32!StubInvoke+0xa7
0012fbcc 7750f15c ole32!CCtxComChnl::ContextInvoke+0xe3
0012fbe8 7750fc79 ole32!MTAInvoke+0x1a
0012fc14 77600e3b ole32!STAInvoke+0x4a
0012fc48 776009bc ole32!AppInvoke+0x7e
0012fd1c 77600df2 ole32!ComInvokeWithLockAndIPID+0x2e0
0012fd48 7750fcb3 ole32!ComInvoke+0x60
0012fd5c 7750fae9 ole32!ThreadDispatch+0x23
0012fd74 77d48744 ole32!ThreadWndProc+0xfe
0012fda0 77d48826 USER32!InternalCallWinProc+0x28
0012fe08 77d489dd USER32!UserCallWinProcCheckWow+0x150
0012fe68 77d496d7 USER32!DispatchMessageWorker+0x306
0012fe78 6600a4a3 USER32!DispatchMessageA+0xf
0012feb8 6600a41a MSVBVM60!ThunderMsgLoop+0xfd
0012fecc 6600a3bc MSVBVM60!CMsoCMHandler::FPushMessageLoop+0x19
0012fefc 6600a2f8 MSVBVM60!SCM::FPushMessageLoop+0xb9
0012ff18 6600a2c3 MSVBVM60!SCM_MsoCompMgr::FPushMessageLoop+0x2b
0012ff3c 6600361c MSVBVM60!CMsoComponent::PushMsgLoop+0x26
0012ffb8 00404dba MSVBVM60!ThunRTMain+0x9b
0012fff0 00000000 AppMain!__vbaS+0xa

Here is the Trace before the module is unloaded:

ChildEBP RetAddr Args to Child
0012ea2c 7c90e96c 7c91e7d3 ffffffff 68640000 ntdll!KiFastSystemCallRet
0012ea30 7c91e7d3 ffffffff 68640000 0012eba4 ntdll!NtUnmapViewOfSection
0012eb20 7c80abf7 68640000 0012ebbc 0012ee74 ntdll!LdrUnloadDll+0x31a
0012eb34 77513442 68640000 0012ee94 77513456 kernel32!FreeLibrary+0x3f
0012eb40 77513456 0012ebc8 776067e0 00000000 ole32!
0012eb54 775135fe 774e1ab0 00000000 00000000 ole32!
0012ee94 77513578 ffffffff 00146080 0fca6b18 ole32!
0012eea4 775133a2 ffffffff 00000000 6605a01e ole32!
0012eeb0 6605a01e 08000000 0fca6a84 0012eee8 ole32!
0012eec4 6605b4d1 00ee5314 00000000 0fc8e094 MSVBVM60!
0012eee8 6601c56a 0fca67b8 00000000 00000000 MSVBVM60!CCreDestroyCtl
0012ef2c 6601bc56 000b154a 00000000 660c9ed5 MSVBVM60!CCreFUnloadForm
0012ef38 660c9ed5 0fc8d8e4 00000009 101f2570 MSVBVM60!CUnkDesk::Release
0012ef50 6600e720 137e1024 77124918 137e1008 MSVBVM60!
0012ef58 77124918 137e1008 0012efc8 0012ef78 MSVBVM60!
0012ef6c 12d75f49 101f2570 0012f020 101f2418 OLEAUT32!VariantClear
0012efc8 12d715d4 0012f084 101f2418 00000000 AppContainer!
_variant_t::~_variant_t+0x29 [c:\program files\microsoft visual studio
\vc98\include\comutil.h @ 1736]
0012f02c 12d74b25 0012f0dc 101f2418 00000000 AppContainer!
std::pair<_bstr_t const ,_variant_t>::~pair<_bstr_t const ,_variant_t>
0012f084 12d749b2 00000000 0012f138 101f2418 AppContainer!
std::pair<_bstr_t const ,_variant_t>::`scalar deleting
0012f0dc 12d73d46 101f2568 0012f1b0 101f2418 AppContainer!std::_Destroy
+0x22 [c:\program files\microsoft visual studio\vc98\include\xmemory @
0012f138 12d72cb5 101f2568 0012f21c 0012f22c AppContainer!

::_Kfn,std::less<_bstr_t>,std::allocator<_variant_t> >::_Destval+0x26

[c:\program files\microsoft visual studio\vc98\include\xtree @ 585]
0012f1bc 12d78f2d 0012f278 101f2418 0012f2a8 AppContainer!

::_Kfn,std::less<_bstr_t>,std::allocator<_variant_t> >::erase+0x825

[c:\program files\microsoft visual studio\vc98\include\xtree @ 359]
0012f21c 12d78bf4 0012f278 101f2558 0012f318 AppContainer!

::erase+0x2d [c:\program files\microsoft visual studio\vc98\include

\map @ 104]
0012f2c0 12d78971 0012f37c 0017a958 00000000 AppContainer!
CPropertyContainer::RemoveMapObject+0x224 [c:\mks\fw13dev\source
\AppContainer\cpropertycontainer.h @ 146]
0012f324 12d79d9d 0012f3e0 0017a958 00000000 AppContainer!
CPropertyContainer::~CPropertyContainer+0x41 [c:\mks\fw13dev\source
\AppContainer\cpropertycontainer.h @ 162]
0012f388 12d77565 0012f448 0017a958 00000000 AppContainer!
+0x6d [c:\program files\microsoft visual studio\vc98\atl\include
\atlcom.h @ 2411]
0012f3e0 12d79e78 00000001 00000000 0017a958 AppContainer!
ATL::CComObject<CPropertyContainer>::`scalar deleting destructor'+0x25
0012f448 79e8dbde 101f2270 7ad569e4 00000008 AppContainer!
ATL::CComObject<CPropertyContainer>::Release+0x48 [c:\program files
\microsoft visual studio\vc98\atl\include\atlcom.h @ 2419]
0012f49c 79e8db4a 101f2270 7ad5699c 00000008 mscorwks!
0012f4e4 79e8dac5 101f2270 0dd8e560 7ad56860 mscorwks!SafeReleaseHelper
0012f518 79f27983 101f2270 0dd8e560 00000001 mscorwks!SafeRelease+0x2f
0012f530 79f2792e 7ad56818 00000001 0dd8e560 mscorwks!
0012f560 79f279dc 0dd8e560 7ad568e8 00000001 mscorwks!
0012f590 79f279b0 01a1fb7c 79f27997 139126a0 mscorwks!RCW::Cleanup
0012f598 79f27997 139126a0 7ad568b0 0012f5e4 mscorwks!
0012f5c8 79f277e5 001466a8 001466a8 0012f5f4 mscorwks!
0012f5d8 77525fbe 01a1fa18 0012f5f8 0012f620 mscorwks!
0012f5f4 77e7a19c 0015b2f8 001d5848 02020202 ole32!
0012f610 77ef321a 77525f83 0012f624 00000002 RPCRT4!Invoke+0x30
0012fa14 77ef3bf3 0015ef78 0015d3a0 001d28bc RPCRT4!NdrStubCall2+0x297
0012fa6c 77600c31 0015ef78 001d28bc 0015d3a0 RPCRT4!
0012faac 77600bdb 001d28bc 001d52e4 00000000 ole32!SyncStubInvoke+0x33
0012faf4 7750f237 001d28bc 0015d2b8 0015ef78 ole32!StubInvoke+0xa7
0012fbcc 7750f15c 0015d3a0 00000000 0015ef78 ole32!
0012fbe8 7750fc79 001d28bc 00000001 0015ef78 ole32!MTAInvoke+0x1a
0012fc14 77600e3b 001d28bc 00000001 0015ef78 ole32!STAInvoke+0x4a
0012fc48 776009bc 001d2868 0015d3a0 0015ef78 ole32!AppInvoke+0x7e
0012fd1c 77600df2 001d2868 0015d600 00000000 ole32!
0012fd48 7750fcb3 001d2868 00000400 001464a8 ole32!ComInvoke+0x60
0012fd5c 7750fae9 001d2868 0012fddc 7750fa56 ole32!ThreadDispatch+0x23
0012fd74 77d48744 003f024a 00146080 0000babe ole32!ThreadWndProc+0xfe
0012fda0 77d48826 7750fa56 003f024a 00000400 USER32!InternalCallWinProc
0012fe08 77d489dd 00000000 7750fa56 003f024a USER32!
0012fe68 77d496d7 0012fe90 00000001 0012feb8 USER32!
0012fe78 6600a4a3 0012fe90 ffffffff 00e0379c USER32!DispatchMessageA
0012feb8 6600a41a ffffffff 00e037c4 00e00000 MSVBVM60!ThunderMsgLoop
0012fecc 6600a3bc 00e0379c ffffffff 00e03894 MSVBVM60!
0012fefc 6600a2f8 00e03894 ffffffff 00000f50 MSVBVM60!
0012ff18 6600a2c3 00e037c0 00e03894 ffffffff MSVBVM60!
0012ff3c 6600361c ffffffff 00e3d0b8 00000018 MSVBVM60!
0012ffb8 00404dba 004051e4 7c816fd7 00e3d0b8 MSVBVM60!ThunRTMain+0x9b
0012fff0 00000000 00404db0 00000000 78746341 AppMain!__vbaS+0xa

On Feb 26, 6:05 pm, "Ivan Brugiolo [MSFT]"
<> wrote:

You're right, I ment DllCanUnloadNow.
If DllCanUnloadNow returns 0, it means it returned S_OK,
it means that the COM infrastructure is free to unload the module.

For a moment, think outside of the concept of `the code I write`,
but, instead, the rules by which the system runs.
If the module count is positive, and your DLL ges unload by
CoFreeUnusedLibraries, then, there's a bug in the system.
Can you track the value of the module count
(you can set a breakpoint on address to do that)
across your repro ?

BTW, the ATL code does have the problem outlined

template <class Base>
class CComObject : public Base
    typedef Base _BaseClass;
    CComObject(void* = NULL)
    // Set refcount to 1 to protect destruction
        m_dwRef = 1L;
        _Module.Unlock(); <<<<-------------- this is executed before
                                      <<<< the destructor

Can you try to do all the cleanup un FinalRelease ?

This posting is provided "AS IS" with no warranties, and confers no
Use of any included script samples are subject to the terms specified

<> wrote in message

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