fo_seek
if ((filepos = (f->f_ops->fo_seek)(f, (off_t)0, SEEK_CUR)) == -1)
return (f->f_ops->fo_seek)(f, offset, where);
off_t (*fo_seek)(struct open_file *, off_t, int);
.fo_seek = dmabuf_seek,
.fo_seek = badfo_seek
.fo_seek = badfo_seek
.fo_seek = badfo_seek
.fo_seek = badfo_seek
.fo_seek = badfo_seek
error = fo_seek(fp, offset, whence, res);
.fo_seek = vn_seek
int (*fo_seek) (struct file *fp, off_t offset, int whence, off_t *res);
return ((*fp->f_ops->fo_seek)(fp, offset, whence, res));
.fo_seek = devfs_fo_seek