Re: COM dll server registration on Windows 64 bits

"Willy Denoyette [MVP]" <>
Mon, 3 Mar 2008 18:11:37 +0100
"Olivier" <> wrote in message

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Hello Willy,

"Willy Denoyette [MVP]" <> a ?crit dans le
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"Olivier" <> wrote in message


We have another question related to this problem:

Can a 64 bits process (or dll) is able to access a TypeLib
embedded in a 32 bits dll (or the opposite)?

Using ProcessExplorer, we saw a 64 bits dll listed in our 32 bits
process (and, yes, we are sure that this dll is 64 bits).

Best regards,


Are you sure that the 64-bit DLL was a COM DLL?
For instance, a 32-bit process loads the 32-bit and the 64-bit of
the ntdll.dll, part of each 32-bit process contains 64-bit code
which is used when calling into the kernel. Besides ntdll.dll there
are other dll's loaded that are 64-bit (wow64.dll, wow64cpu.dll

Yes, we are sure because this is one of our COM dlls compiled in 64
bits :)
ProcessExplorer and DependencyWalker tell us that this dll is 64

Nevertheless, I will check this again.

Maybe a typelib resource can be accessed using RPC? I mean if a 32
bits process requires this typelib (only) stored in a 64 bits
module, then COM may use RPC to access it (or a specific Win32 API
that is only interested by the resource)

Does COM exist as 32 bits and 64 bits on Windows64?

I think so.


I checked this again and I'm sure I'm correct.

No doubt, I believe you are correct.

On the 64 bits dll listed as belonging to our 32 bits process,
ProcessExplorer tells me that the 'Mapping Type' is 'Data'
On another 32 bits dll (in fact the same dll compiled as 32 bits)
listed also in the 32 bits process, ProcessExplorer tells me that the
'Maping Type' is 'Image' (I think that means 'Code')

That's right, "Image" is the mapping type for code segments.

That would confirm that Win64 is able to load data (like resources)
stored in a 64 bits dll into a 32 bits process?

Sure, the OS can read whatever "file", but the OS loader will not
"load" a 64-bit module in a 32-bit process (as a result of a
LoadLibrary(Ex) call).

This seems correct for me, since, in this case, data (typelib
resource) does not need the code stored in the dll :)

Why would he need to load a 64-bit COM DLL, when the typelib info is
already present in the loaded 32-bit module?

Because registry tells to COM where the required typelib is located.
See my previous posts, above in this thread, about order of
registration of a COM dll compiled both as 32 and 64 bits. As I learned
it, the "last writer wins".

In this scenario, the 64 bits version of our dll was the last
registered. I mean, I registered the 32 bits version and the 64 bits
version of the dll, in this order. If I register the 32 bits version in
last, then the 64 bits version is no more listed as belonging to our 32
bits process. In this case, there is only the 32 bits version.

There must be another reason why the 64-bit DLL is mapped into the
32-bit process, I would love to repro this case but I can't :-(.

No, I think that the (only) reason is that the required resource
(typelib) is embedded in the 64 bits dll.

I wonder how you built the DLL and what version of the OS and VS you
are using.

32 bits version is compiled using VC++6.0 on Windows XP Pro 64 bits
64 bits version is compiled using VC++2005 on Windows XP Pro 64 bits
(same seat)

Interesting, does not it?

Sure it is, however I don't like the idea of wasting VAS because some
libraries (related but not the same) get mapped twice in the same
process. Imagine how much memory space you waste in a (32-bit!)
process that loads a bunch of COM server DLL's...

Indeed, I agree with you. BTW, I should mention that our product is
currently a mix of 32 and 64 bits COM modules. They interoperate
through IPC (when bit model differ). To achieve this, we have created
our own "surrogate process" because we also need to detect at runtime
which version (bit model) of a COM dll we need to use. Since our
application is still 32 bits, sometimes we don't use our surrogate
infrastructure (because the bit model does not differ) and sometimes we
use it. That may explain the mix shown by ProcessExplorer.

As an example: In one of our COM modules, we need to access a MS Access
database. When compiled as 32 bits, there is no problem to do this in
this COM module. When compiled as 64 bits, we use our surrogate
infrastructure to use the 32 bits version of MDAC/ADO, because the
OleDB MS/Jet driver is not available in 64 bits (except on Windows
Server 2003 x64, and, AFAIK, there is no redistribuable package for
others x64 systems!).


Running Vista and WS2008 both 64 bit. Whatever the order of
registration, I only see a single DLL loaded.
I will now try to call an out-proc server (surrogate proc) and see
whether the marshaler is loading the typelib from the last registered


Did some more tests (in-proc, out-proc, COM+ etc) but I can't see any
64-bit dll's getting loaded in a 32-proc.
Also, the typelib's are registered, for each 32 and 64 bit, under the
same key (the typelib UUID).
After registering a 32-bit DLL I see this :
with a REG_SZ that looks like:


After registering the 64-bit version on the same component (same GUID's),
I have a second key under the typelib key.
with a REG_SZ that looks like:

that means that both win32 and a win64 typelib's are getting registered,
each containing the path of the COM server DLL.

This under Vista-64 and WS2008, don't know if this differs from 64-bit


Hello Willy,

Thank you very much for your tests.

Indeed, your results are very interesting because, using Windows XP 64
bits and VisualC++2005, there is no win64 sub key for the typelib
registration (only win32).

Indeed, I ran the same test on XP64, and effectively the sub key does not
get created by regsvr32, which is quite normal as COM itself doesn't know
about this subkey.

So, this seems different when using VS2008. I almost sure that Vista does
not play any role here. You should obtain same results as on Vista64 when
using VS2008 on WinXP64

No at all, VS2008 has nothing to do with this, actually I don't even use
VS2008 for this, I register the component using regsvr32.exe from the
command line and it's regsvr32 which add this subkey.
The version of regsvr32 on Vista-64 (SP1) and WS2008 differs from the W2K3
and XP version, so does COM itself (ole32.dll).

Again, for me, there is no "real" problem in loading a 64 bits dll as a
*data file* in a 32 bits process (I think this is what COM is doing when
looking for a typelib resource). As you already mentioned it, the ONLY
problem is that, depending of order of registration, sometimes we have
only the 32 bits version of the module and sometimes we have a mix of

Maybe there is no "real" problem with this, but, there must be a reason why
MS has changed the typelib registration/loading on the latest OS'ses. My
guess is that they too didn' like ;-) the extra resource consumption related
to the loading of the "latest registered" typelib.


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