+
+/*
+ * Rearange an mbuf chain so that len bytes are contiguous
+ * and in the data area of an mbuf (so that mtod and dtom
+ * will work for a structure of size len). Returns the resulting
+ * mbuf chain on success, frees it and returns null on failure.
+ * If there is room, it will add up to max_protohdr-len extra bytes to the
+ * contiguous region in an attempt to avoid being called next time.
+ */
+int MPFail;
+
+struct mbuf *
+m_pullup(n, len)
+ register struct mbuf *n;
+ int len;
+{
+ register struct mbuf *m;
+ register int count;
+ int space;
+
+ /*
+ * If first mbuf has no cluster, and has room for len bytes
+ * without shifting current data, pullup into it,
+ * otherwise allocate a new mbuf to prepend to the chain.
+ */
+ if ((n->m_flags & M_EXT) == 0 &&
+ n->m_data + len < &n->m_dat[MLEN] && n->m_next) {
+ if (n->m_len >= len)
+ return (n);
+ m = n;
+ n = n->m_next;
+ len -= m->m_len;
+ } else {
+ if (len > MHLEN)
+ goto bad;
+ MGET(m, M_DONTWAIT, n->m_type);
+ if (m == 0)
+ goto bad;
+ m->m_len = 0;
+ if (n->m_flags & M_PKTHDR)
+ M_COPY_PKTHDR(m, n);
+ }
+ space = &m->m_dat[MLEN] - (m->m_data + m->m_len);
+ do {
+ count = min(min(max(len, max_protohdr), space), n->m_len);
+ bcopy(mtod(n, caddr_t), mtod(m, caddr_t) + m->m_len,
+ (unsigned)count);
+ len -= count;
+ m->m_len += count;
+ n->m_len -= count;
+ space -= count;
+ if (n->m_len)
+ n->m_data += count;
+ else
+ n = m_free(n);
+ } while (len > 0 && n);
+ if (len > 0) {
+ (void) m_free(m);
+ goto bad;
+ }
+ m->m_next = n;
+ return (m);
+bad:
+ m_freem(n);
+ MPFail++;
+ return (0);
+}