How to store a pointer to the callback function in the shared block of data in the DLL?

I'm tring to write a C DLL that will call a function in VB when certain conditions are met. I need to store that pointer to the callback function somewhere in the DLL, and that pointer must be accessible to all the instances of the DLL. Since the DLL is accessed by both an exe and VB application (they are actually plug-ins in exe the host aplication), there can be quote a few of them running at the same time.

Thanx in advance for all your help.