switch_fn
r = pb->switch_fn(pb->current_child, cb->bus_number, pb->switch_data);
int (*switch_fn)(int cur, int desired, void *data),
pb->switch_fn = switch_fn;
int (*switch_fn)(int current_child, int desired_child, void *data);
r = pb->switch_fn(pb->current_child, cb->bus_number, pb->switch_data);
r = pb->switch_fn(pb->current_child, cb->bus_number, pb->switch_data);
r = pb->switch_fn(pb->current_child, cb->bus_number, pb->switch_data);
int (*switch_fn) (int cur, int desired, void *data),
makecontext(&uc, switch_fn, 0);