shm_alloc
shm = shm_alloc(td->td_ucred, O_RDWR, false);
shmfd = shm_alloc(td->td_ucred, cmode, largepage);
shmfd = shm_alloc(td->td_ucred, cmode,
struct shmfd *shm_alloc(struct ucred *ucred, mode_t mode, bool largepage);