Hi,
I already posted a related question on the comtypes mailing list but
unfortunately Thomas Heller told me that he cannot actively provide support
anymore.
So I'm hoping to get some help here because it might also relate more to ctypes.
At first, sorry for the long mail but it's a rather complex issue and I want to
provide as much information and context as required.
Heres the problem:
We wrote a COM server using comtypes which worked well on Win XP. However, on
Windows 2008 and Win 7, the same code crashes after calling a specific method
that our server provides.
By "crashes" I mean the COM client (Tried it with a C++ and python client)
receives a COMError:
_ctypes.COMError: (-2147417851, 'Ausnahmefehler des Servers.', (None, None,
None, 0, None))
The Server does *not* throw any exceptions as far as I can see and keeps on
running. I added some printf debugging in comtypes but everything seems fine
and ends up calling some win32api functions after message dispatching is done.
Every time this happens, in the windows application event log, there is an
entry saying "python.exe" was the offending application, "ntdll.dll" was the
offending module. I also set up windows debugging modules with shows the
following stack trace (just first few lines at the top):
ntdll!RtlpLowFragHeapFree+0x31 (FPO: [Non-Fpo])
01 0021ed04 76f49c46 00390000 00000000 029e76e0 ntdll!RtlFreeHeap+0x101
02 0021ed18 7718afbd 00390000 00000000 029e76e8 kernel32!HeapFree+0x14
03 0021ed2c 7718afd9 7725f6f8 029e76e8 0021ed54
ole32!CRetailMalloc_Free+0x1c (FPO: [Non-Fpo])
04 0021ed3c 75be6efc 029e76e8 01f685fe 0021edcc ole32!CoTaskMemFree+0x13
Since the problem arises reproducibly when calling one method the has a pointer
to an array as input and output, I believe this is some kind of memory issue
(which also the stack trace suggests) that was probably just not
recognized/caught by WinXP before but now shows up in more recent windows
versions.
My assumption is, that either I need to actively allocate or deallocate memory
for in/out parameters.
This is the method signature of the method that our server provides and that
crashes:
IOPCItemMgt::ValidateItems
HRESULT ValidateItems(
[in] DWORD dwCount,
[in, size_is(dwCount)] OPCITEMDEF * pItemArray,
[in] BOOL bBlobUpdate,
[out, size_is(,dwCount)] OPCITEMRESULT ** ppValidationResults,
[out, size_is(,dwCount)] HRESULT ** ppErrors
);
Assume that this is the signature in python comtypes server:
def ValidateItems(self, count, p_item_array, update_blob):
and this is the ctypes structure which should be returned in "
ppValidationResults ":
OpcDa.tagOPCITEMRESULT
How would I create these structures (especially ppValidationResults), its
contents and the pointers so that I would not create any memory issues, null
pointers or whatsoever?
My implementation creates a few items by calling
validation_result = OpcDa.tagOPCITEMRESULT()
filling in the contents with e.g.
validation_result.hServer = server_item_handle
adding it to a list
validation_results.append(validation_result)
then converting it to a pointer to an array with a helper function:
def p_list(items):
c_items = (type(items[0])*len(items))(*items)
p_items = cast(c_items, POINTER(type(items[0])))
return p_items
(...)
p_validation_results = p_list(validation_results)
doing the same with "errors" and then returning it:
return p_validation_results, p_errors
Is that correct?
Do I have to keep references to the list, the pointer or the item strcuture
objects to avoid them beeing garbage collected?
Thanks a lot!
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