How to write the body for the call back function in a mcaro?

Question

I have a macro as below

#define _UPDATE(layer, fb_pointer) (_update(((void*)fb_pointer)))

the above line is present in _update.cfg file

so i am calling the macro as below:

UINT8 fl_status_U8 = _UPDATE(fl_lid_U8, _GET_WORKING_FB_PTR(fl_lid_U8));

above line is in _update.c

So far everything is going well. Now where should i write the body of the call back function _update(void * ptr)?

I am writing a simulation in vc++ express edition visual studio 2008.