Is it possible to access a 64-bit dll from a 32-bit application?

Question

I have a Delphi application similar to Taskbar Shuffle that includes a hook dll.

EDIT: This hook DLL communicates with the main app by sending windows messages.

I want to add support to XP and Vista x64 and my initial idea was to convert the dll to 64-bit (compiling it with FreePascal) but keep the application 32-bit for now (Delphi).

Is it possible for a 32-bit application to access a 64-bit dll?

EDIT2: I'm loading the dll via LoadLibrary so I guess I'm stuck since a 32-bit process won't be able to load a 64-bit dll, according with what I read on the link pointed by Lars Truijens on one of the answers below.

Answer 1

As long as the 64-bit DLL is being loaded by a separate 64-bit process, and all communication between 32-bit process and 64-bit DLL is via loose-coupled IPC-like mechanisms that the OS can marshall, then yes you can do that.

I've done something similar. A 32-bit application needed a custom Print Spooler add-in implemented in a DLL. The app and the spooler add-in communicated via IPC mechanisms (a fancy way of saying temporary files, in this case).

On 64-bit systems everything about the 32-bit application worked fine except that the Print Spooler refused to load the add-in DLL, because the Print Spooler was of course a 64-bit process.

The solution was as simple as rebuilding only the Spooler add-in in 64-bit. No need to change the entire 32-bit app.

Answer 2

No. You'll have to compile two versions: 64-bit and 32-bit.