sbsndptr_noadv
mb = sbsndptr_noadv(&so->so_snd, off, &moff);
mb = sbsndptr_noadv(sb, sb_offset, &moff);
mb = sbsndptr_noadv(sb, sb_offset, &moff);
sbsndptr_noadv(struct sockbuf *sb, u_int off, u_int *moff);