M_RDONLY
#define M_WRITABLE(m) (!((m)->m_flags & M_RDONLY) && \
(M_PKTHDR|M_EOR|M_RDONLY|M_BCAST|M_MCAST|M_PROMISC|M_VLANTAG| \
n->m_flags |= m->m_flags & M_RDONLY;
m->m_flags = m->m_flags & (M_EXT | M_RDONLY | M_NOFREE | flags);
n->m_flags &= ~M_RDONLY;