In this example:

typedef void (*callback_t)(void *arg);

    typedef struct first_handler {
         callback_t cb;
    } first_handler_t;

    typedef struct second_handler {
         callback_t cb;
    } second_handler_t;


Jextract ends up generating TWO identical functional interfaces, one inside first_handler and another inside second_handler.

There is no functional interface at the toplevel either, which is odd, given that the function pointer typedef belongs to the toplevel.