According to the game maker documentation for dll manipulation:
Returns the address of a GameMaker function. This can be passed to DLL's so the can call directly into GameMaker.
This feature is unsuported, so care must be taken when using this function.
Please Note: These functions use Delphi 2010 PASCAL calling convention and string formats.
Let's assume one writes a C++ dll and wants to use those function pointers to make game maker function calls within the dll code.
What does one cast the function pointer to? I am familiar with passing a C function pointer to a C++ dll (it is blatantly obvious what you cast the pointer to; the same thing as what you would within the C code). However, I do not know what I would cast functions to coming from game maker. I imagine it is a relatively "simple" thing to do once you know the magic dance. It just doesn't say anywhere what to do.
I think the page might be trying to tell me that this is equivalent to passing a pascal function pointer to a C++ dll. I don't really know. I can't be the only guy with this problem.