C/C++ :: Indices Returned In Bsearch

I have some struct which contains: void *elems (basically a pointer to an array of contiguous memory).

I want to use bsearch to return a pointer, and then somehow figure out where in the array that value is. Having a pointer in this case isn't enough, I need to know what the index is. I've tried a number of things:

int index;
void *value = bsearch(key, start_ptr, cv->count, cv->elemsz, cmp);
index = &value - &start_ptr;
return index; [
Replacing the second line with:

// in the first instance
index = (char*) value - (char*) start_ptr;

// in other instances...
index = ((char*) value - start_ptr))/cv->elemsz)

But nothing works.