last_ctl_resid
if ((g->mt_resid = (short)softc->last_ctl_resid) != 0) {
softc->last_ctl_resid = 0;
sep->ctl_resid = softc->last_ctl_resid;
softc->last_ctl_resid = 0;
softc->last_ctl_resid = resid;
softc->last_ctl_resid = 0;
softc->fileno += (count - softc->last_ctl_resid);
softc->blkno += (count - softc->last_ctl_resid);
if (softc->last_ctl_resid || softc->blkno < 0) {
softc->last_ctl_resid = 0;
nwm = nmarks - softc->last_ctl_resid;