ctypes: reference of a struct member?

[ma]

If I have this struct in C:

struct spam {
     int ham;
  char foo;
};


if I have this declaration:
struct spam s_;

If I wanted to pass a reference to a function of s_'s foo character, I
can do something like this:

somefunc(&s_.foo)

How do I do the same thing in ctypes?
ctypes.addressof(s_) + ctypes.sizeof(spam.foo)*(1)