64 bit DLLs in Windows

[John E]

I'm sure this might seem like a dumb question one way or the other - but if I build a 32-bit app which somebody uses on Win64, can it use a 64-bit version of a third party DLL or does the whole chain need to be 32-bit? I'm assuming I can't mix & match because presumably, the app and all its dependencies would need to run in a 32-bit VM. Is that the case?

And what about the other way around? Can a 64-bit app use a 32-bit third party DLL or does the whole chain need to be 64-bit??

[Paul McKenzie]

I'm sure this might seem like a dumb question one way or the other - but if I build a 32-bit app which somebody uses on Win64, can it use a 64-bit version of a third party DLL

No.

It is easy to test:

#include <windows>

int main()
{
     HMODULE hMod = LoadLibrary( "MyDLL" );
     if ( hMod )
    {
           // Loaded successfully
          FreeLibrary( hMod );
    }
    else
    {
        // Didn't load successfully
    }
}

Assume this is a 32-bit application and MyDLL is a 64-bit DLL. What will happen (assuming that MyDLL would have loaded OK if the app was also 64-bit instead of 32-bit)?

Then make the same test vice-versa -- the app is 64-bit and the DLL is 32-bit.

And what about the other way around? Can a 64-bit app use a 32-bit third party DLL

No. The closest you can do is to launch a 32-bit process that uses the 32-bit DLL and then utilize some sort of IPC to communicate the results/data/whatever back to the 64-bit app.

Regards,

Paul McKenzie

[John E]

Thanks Paul. That's pretty much what I thought.

[John E]

Having looked into this a bit further I've noticed that it's becoming common on Linux for libraries to be distributed in "mixed 32/64" packages. From what I can gather, each module (lib / so) contains functions compiled both in 32-bit form and also 64 bit - so the same physical module can be used with either a 32-bit version of the app or a 64-bit version. I can (kinda) see the advantage of that but I'm guessing we can't do that with VC++?