How to access members through a void pointer

Question

Started by trying to write a small program to translate basic arithmetic into English, I end up building a binary tree(which is inevitably very unbalanced) to represent the

Answer 1

Just convert p to the relevant pointer type:

s *a = p;

a->i1 = 42;
a->i2 = 31;

or

((s *) p)->i1 = 42;
((s *) p)->i2 = 31;