Given a pointer to member a within a struct, write a routine that returns a pointer to the struct

Here is an interview question that I saw on some forum. I've been trying to figure out how it works but I don't quite get it. Could somebody explain how it works?

Q: Given a pointer to member a within a struct, write a routine that returns a pointer to the struct.

struct s 
{
   ...
   int a;
   …
};

struct s *get_s_ptr(int *a_ptr)
{
   // implement this.
}

The answer is:

struct s* get_s_ptr(int *a_ptr)
{
   return (struct s*)((char*)a_ptr - (int)&((struct s*)0)->a);
}