kern_sigaltstack
(void)kern_sigaltstack(td, &ss, NULL);
error = kern_sigaltstack(td, ssp, &oss);
error = kern_sigaltstack(td, (uap->uss != NULL) ? &ss : NULL,
(void)kern_sigaltstack(td, &ss, NULL);
error = kern_sigaltstack(td, (uap->ss != NULL) ? &ss : NULL,
int kern_sigaltstack(struct thread *td, stack_t *ss, stack_t *oss);