C/C++ :: Retrieve Index After Bsearch Or Lfind?

Basically, I'm trying to figure out the index in an array from a pointer that is returned by either bsearch or lfind.

void *val;
void *begin = (char *)v->elems (I need to use this separate variable)

Then, I call on either search, which seems to be working fine...

val = bsearch(key, begin, v->count, v->elemsz, cmp);

I'm trying to do the following:

index = *(int*)((char*)value_to_find - (char*)start_ptr)/sizeof(cv->elemsz)

However it segfaults everytime I do this. My logic is that I cast both void pointers to type of char*, then subtract the distance b/w the pointers since you can't do pointer arithmetic on void. I divide this by the size of each element to return the index, which needs to be cast to an int. What am I missing here?