[a1369209993]

To be fair, size_t doesn't solve this particular problem; you also need to use correct array slice representation (ptr,len) not (start,end), and calculate the midpoint accordingly (ie (ptr,len/2) or (ptr+len/2,len-len/2)).

(And because C doesn't mandate correct handling of benign undefined behavior, you still have a problem if you `return ptr-orig_ptr` as a size_t offset (rather than returning the final ptr directly), because pointer subtraction is specified as producing ptrdiff_t (rather than size_t), which can 'overflow' for large arrays, despite that it's immediatedly converted back to a correct value of size_t.)