---> struct foo2
  |        |
  |        |
  |        V
  |  void (*func1)(struct foo1*) <----
  |        |                         |
1 |        |                         |
  |        V                         |
  |  struct foo1*                    | 
  |        |                         | 2
  |        |                         |
  |        V                         |
  |  struct foo1                     |
  |        |                         |
  |________|                         |
           |                         |
           |                         |
     struct foo3*                    |
           |                         |
           |                         | 
     struct foo                      |
           |                         |
           |_________________________|