Combine 32- and 64bit DLLs in one program

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On 64-bit Windows 64-bit processes can not use 32-bit DLLs and 32-bit processes can’t use 64-bit DLLs. Microsoft has documented this:

On 64-bit Windows, a 64-bit process cannot load a 32-bit dynamic-link library (DLL). Additionally, a 32-bit process cannot load a 64-bit DLL.

You would need a 32-bit process that communicates with the 32-bit DLL and a 64-bit process to communicate with the 64-bit DLL. Microsoft says this:

However, 64-bit Windows supports remote procedure calls (RPC) between 64-bit and 32-bit processes (both on the same computer and across computers).

The problem then becomes one of how to have Python communicate with these processes. Some form of Interprocess Communication (IPC) would be needed. Microsoft created a technology decades ago that can do just that – COM interfaces using Out of Process COM servers (out-of-proc).

The general idea is:

  • Create a 64-bit out-of-proc COM server that wraps (and exposes) the needed methods and data of the 64-bit DLL.
  • Create a 32-bit out-of-proc COM server that wraps (and exposes) the needed methods and data of the 32-bit DLL.
  • Write either 32-bit or 64-bit client code that instantiates the COM objects and calls their interfaces. Python can be used as a COM client via win32com

COM provides an IPC mechanism under the hood that allows a 64-bit client to access a 64-bit out-of-proc COM server and for a 64-bit client to access a 32-bit out-of-proc server. You can even have 32-bit clients communicate with 32-bit and 64-bit out-of-proc COM servers as well.

I haven’t done low level Windows work using the newer MS languages. When I had to do what you needed in your question the two main technologies that made it easy to write COM servers and COM interfaces were:

I had a preference for ATL since it didn’t require the MFC library and had less overhead.

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