pbuf_ref(struct pbuf *p)
{
  SYS_ARCH_DECL_PROTECT(old_level);
  /* pbuf given? */
  if (p != NULL) {
    SYS_ARCH_PROTECT(old_level);
    ++(p->ref);
    SYS_ARCH_UNPROTECT(old_level);
  }
}

/**
 * Concatenate two pbufs (each may be a pbuf chain) and take over
 * the caller's reference of the tail pbuf.
 * 
 * @note The caller MAY NOT reference the tail pbuf afterwards.
 * Use pbuf_chain() for that purpose.
 * 
 * @see pbuf_chain()
 */

void
pbuf_cat(struct pbuf *h, struct pbuf *t)
{
  struct pbuf *p;

  LWIP_ASSERT("h != NULL (programmer violates API)", h != NULL);
  LWIP_ASSERT("t != NULL (programmer violates API)", t != NULL);
  if ((h == NULL) || (t == NULL)) return;

  /* proceed to last pbuf of chain */
  for (p = h; p->next != NULL; p = p->next) {
    /* add total length of second chain to all totals of first chain */
    p->tot_len += t->tot_len;
  }
  /* { p is last pbuf of first h chain, p->next == NULL } */
  LWIP_ASSERT("p->tot_len == p->len (of last pbuf in chain)", p->tot_len == p->len);
  LWIP_ASSERT("p->next == NULL", p->next == NULL);
  /* add total length of second chain to last pbuf total of first chain */
  p->tot_len += t->tot_len;
  /* chain last pbuf of head (p) with first of tail (t) */
  p->next = t;
  /* p->next now references t, but the caller will drop its reference to t,
   * so netto there is no change to the reference count of t.
   */
}

/**
 * Chain two pbufs (or pbuf chains) together.
 * 
 * The caller MUST call pbuf_free(t) once it has stopped
 * using it. Use pbuf_cat() instead if you no longer use t.
 * 
 * @param h head pbuf (chain)
 * @param t tail pbuf (chain)
 * @note The pbufs MUST belong to the same packet.
 * @note MAY NOT be called on a packet queue.
 *
 * The ->tot_len fields of all pbufs of the head chain are adjusted.
 * The ->next field of the last pbuf of the head chain is adjusted.
 * The ->ref field of the first pbuf of the tail chain is adjusted.
 *
 */
void
pbuf_chain(struct pbuf *h, struct pbuf *t)
{
  pbuf_cat(h, t);
  /* t is now referenced by h */
  pbuf_ref(t);
  LWIP_DEBUGF(PBUF_DEBUG | DBG_FRESH | 2, ("pbuf_chain: %p references %p\n", (void *)h, (void *)t));
}

/* For packet queueing. Note that queued packets MUST be dequeued first
 * using pbuf_dequeue() before calling other pbuf_() functions. */
#if ARP_QUEUEING
/**
 * Add a packet to the end of a queue.
 *
 * @param q pointer to first packet on the queue
 * @param n packet to be queued
 *
 * Both packets MUST be given, and must be different.
 */
void
pbuf_queue(struct pbuf *p, struct pbuf *n)
{
#if PBUF_DEBUG /* remember head of queue */
  struct pbuf *q = p;
#endif
  /* programmer stupidity checks */
  LWIP_ASSERT("p == NULL in pbuf_queue: this indicates a programmer error\n", p != NULL);
  LWIP_ASSERT("n == NULL in pbuf_queue: this indicates a programmer error\n", n != NULL);
  LWIP_ASSERT("p == n in pbuf_queue: this indicates a programmer error\n", p != n);
  if ((p == NULL) || (n == NULL) || (p == n)){
    LWIP_DEBUGF(PBUF_DEBUG | DBG_HALT | 3, ("pbuf_queue: programmer argument error\n"))
    return;
  }

  /* iterate through all packets on queue */
  while (p->next != NULL) {
/* be very picky about pbuf chain correctness */
#if PBUF_DEBUG
    /* iterate through all pbufs in packet */
    while (p->tot_len != p->len) {
      /* make sure invariant condition holds */
      LWIP_ASSERT("p->len < p->tot_len", p->len < p->tot_len);
      /* make sure each packet is complete */
      LWIP_ASSERT("p->next != NULL", p->next != NULL);
      p = p->next;
      /* { p->tot_len == p->len => p is last pbuf of a packet } */
    }
    /* { p is last pbuf of a packet } */
    /* proceed to next packet on queue */
#endif
    /* proceed to next pbuf */
    if (p->next != NULL) p = p->next;
  }
  /* { p->tot_len == p->len and p->next == NULL } ==>
   * { p is last pbuf of last packet on queue } */
  /* chain last pbuf of queue with n */
  p->next = n;
  /* n is now referenced to by the (packet p in the) queue */
  pbuf_ref(n);
#if PBUF_DEBUG
  LWIP_DEBUGF(PBUF_DEBUG | DBG_FRESH | 2,
    ("pbuf_queue: newly queued packet %p sits after packet %p in queue %p\n",
    (void *)n, (void *)p, (void *)q));
#endif
}

/**
 * Remove a packet from the head of a queue.
 *
 * The caller MUST reference the remainder of the queue (as returned). The
 * caller MUST NOT call pbuf_ref() as it implicitly takes over the reference
 * from p.
 * 
 * @param p pointer to first packet on the queue which will be dequeued.
 * @return first packet on the remaining queue (NULL if no further packets).
 *
 */
struct pbuf *
pbuf_dequeue(struct pbuf *p)
{
  struct pbuf *q;
  LWIP_ASSERT("p != NULL", p != NULL);

  /* iterate through all pbufs in packet p */
  while (p->tot_len != p->len) {
    /* make sure invariant condition holds */
    LWIP_ASSERT("p->len < p->tot_len", p->len < p->tot_len);
    /* make sure each packet is complete */
    LWIP_ASSERT("p->next != NULL", p->next != NULL);
    p = p->next;
  }
  /* { p->tot_len == p->len } => p is the last pbuf of the first packet */
  /* remember next packet on queue in q */
  q = p->next;
  /* dequeue packet p from queue */
  p->next = NULL;
  /* any next packet on queue? */
  if (q != NULL) {
    /* although q is no longer referenced by p, it MUST be referenced by
     * the caller, who is maintaining this packet queue. So, we do not call
     * pbuf_free(q) here, resulting in an implicit pbuf_ref(q) for the caller. */
    LWIP_DEBUGF(PBUF_DEBUG | DBG_FRESH | 2, ("pbuf_dequeue: first remaining packet on queue is %p\n", (void *)q));
  } else {
    LWIP_DEBUGF(PBUF_DEBUG | DBG_FRESH | 2, ("pbuf_dequeue: no further packets on queue\n"));
  }
  return q;
}
#endif

/**
 *
 * Create PBUF_POOL (or PBUF_RAM) copies of PBUF_REF pbufs.
 *
 * Used to queue packets on behalf of the lwIP stack, such as
 * ARP based queueing.
 *
 * Go through a pbuf chain and replace any PBUF_REF buffers
 * with PBUF_POOL (or PBUF_RAM) pbufs, each taking a copy of
 * the referenced data.
 *
 * @note You MUST explicitly use p = pbuf_take(p);
 * The pbuf you give as argument, may have been replaced
 * by a (differently located) copy through pbuf_take()!
 *
 * @note Any replaced pbufs will be freed through pbuf_free().
 * This may deallocate them if they become no longer referenced.
 *
 * @param p Head of pbuf chain to process
 *
 * @return Pointer to head of pbuf chain
 */
struct pbuf *
pbuf_take(struct pbuf *p)
{
  struct pbuf *q , *prev, *head;
  LWIP_ASSERT("pbuf_take: p != NULL\n", p != NULL);
  LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 3, ("pbuf_take(%p)\n", (void*)p));

  prev = NULL;
  head = p;
  /* iterate through pbuf chain */
  do
  {
    /* pbuf is of type PBUF_REF? */
    if (p->flags == PBUF_FLAG_REF) {
      LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE, ("pbuf_take: encountered PBUF_REF %p\n", (void *)p));
      /* allocate a pbuf (w/ payload) fully in RAM */
      /* PBUF_POOL buffers are faster if we can use them */
      if (p->len <= PBUF_POOL_BUFSIZE) {
        q = pbuf_alloc(PBUF_RAW, p->len, PBUF_POOL);
        if (q == NULL) {
          LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: Could not allocate PBUF_POOL\n"));
        }
      } else {
        /* no replacement pbuf yet */
        q = NULL;
        LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: PBUF_POOL too small to replace PBUF_REF\n"));
      }
      /* no (large enough) PBUF_POOL was available? retry with PBUF_RAM */
      if (q == NULL) {
        q = pbuf_alloc(PBUF_RAW, p->len, PBUF_RAM);
        if (q == NULL) {
          LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: Could not allocate PBUF_RAM\n"));
        }
      }
      /* replacement pbuf could be allocated? */
      if (q != NULL)
      {
        /* copy p to q */
        /* copy successor */
        q->next = p->next;
        /* remove linkage from original pbuf */
        p->next = NULL;
        /* remove linkage to original pbuf */
        if (prev != NULL) {
          /* prev->next == p at this point */
          LWIP_ASSERT("prev->next == p", prev->next == p);
          /* break chain and insert new pbuf instead */
          prev->next = q;
        /* prev == NULL, so we replaced the head pbuf of the chain */
        } else {
          head = q;
        }
        /* copy pbuf payload */
        memcpy(q->payload, p->payload, p->len);
        q->tot_len = p->tot_len;
        q->len = p->len;
        /* in case p was the first pbuf, it is no longer refered to by
         * our caller, as the caller MUST do p = pbuf_take(p);
         * in case p was not the first pbuf, it is no longer refered to
         * by prev. we can safely free the pbuf here.
         * (note that we have set p->next to NULL already so that
         * we will not free the rest of the chain by accident.)
         */
        pbuf_free(p);
        /* do not copy ref, since someone else might be using the old buffer */
        LWIP_DEBUGF(PBUF_DEBUG, ("pbuf_take: replaced PBUF_REF %p with %p\n", (void *)p, (void *)q));
        p = q;
      } else {
        /* deallocate chain */
        pbuf_free(head);
        LWIP_DEBUGF(PBUF_DEBUG | 2, ("pbuf_take: failed to allocate replacement pbuf for %p\n", (void *)p));
        return NULL;
      }
    /* p->flags != PBUF_FLAG_REF */
    } else {
      LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 1, ("pbuf_take: skipping pbuf not of type PBUF_REF\n"));
    }
    /* remember this pbuf */
    prev = p;
    /* proceed to next pbuf in original chain */
    p = p->next;
  } while (p);
  LWIP_DEBUGF(PBUF_DEBUG | DBG_TRACE | 1, ("pbuf_take: end of chain reached.\n"));

  return head;
}

/**
 * Dechains the first pbuf from its succeeding pbufs in the chain.
 *
 * Makes p->tot_len field equal to p->len.
 * @param p pbuf to dechain
 * @return remainder of the pbuf chain, or NULL if it was de-allocated.
 * @note May not be called on a packet queue.
 */
struct pbuf *
pbuf_dechain(struct pbuf *p)
{
  struct pbuf *q;
  u8_t tail_gone = 1;
  /* tail */
  q = p->next;
  /* pbuf has successor in chain? */
  if (q != NULL) {
    /* assert tot_len invariant: (p->tot_len == p->len + (p->next? p->next->tot_len: 0) */
    LWIP_ASSERT("p->tot_len == p->len + q->tot_len", q->tot_len == p->tot_len - p->len);
    /* enforce invariant if assertion is disabled */
    q->tot_len = p->tot_len - p->len;
    /* decouple pbuf from remainder */
    p->next = NULL;
    /* total length of pbuf p is its own length only */
    p->tot_len = p->len;
    /* q is no longer referenced by p, free it */
    LWIP_DEBUGF(PBUF_DEBUG | DBG_STATE, ("pbuf_dechain: unreferencing %p\n", (void *)q));
    tail_gone = pbuf_free(q);
    if (tail_gone > 0) {
      LWIP_DEBUGF(PBUF_DEBUG | DBG_STATE,
                  ("pbuf_dechain: deallocated %p (as it is no longer referenced)\n", (void *)q));
    }
    /* return remaining tail or NULL if deallocated */
  }
  /* assert tot_len invariant: (p->tot_len == p->len + (p->next? p->next->tot_len: 0) */
  LWIP_ASSERT("p->tot_len == p->len", p->tot_len == p->len);
  return (tail_gone > 0? NULL: q);
}