pbuf.c

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/*
 * Copyright (c) 2001-2003 Swedish Institute of Computer Science.
 * All rights reserved. 
 * 
 * Redistribution and use in source and binary forms, with or without modification, 
 * are permitted provided that the following conditions are met:
 *
 * 1. Redistributions of source code must retain the above copyright notice,
 *    this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright notice,
 *    this list of conditions and the following disclaimer in the documentation
 *    and/or other materials provided with the distribution.
 * 3. The name of the author may not be used to endorse or promote products
 *    derived from this software without specific prior written permission. 
 *
 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR IMPLIED 
 * WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF 
 * MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT 
 * SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, 
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT 
 * OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
 * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
 * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING 
 * IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY 
 * OF SUCH DAMAGE.
 *
 * This file is part of the lwIP TCP/IP stack.
 * 
 * Author: Adam Dunkels <adam@sics.se>
 *
 * CHANGELOG: this file has been modified by Sergio Perez Alcañiz <serpeal@disca.upv.es> 
 *            Departamento de Informática de Sistemas y Computadores          
 *            Universidad Politécnica de Valencia                             
 *            Valencia (Spain)    
 *            Date: April 2003                                          
 *  
 */

#include "lwip/opt.h"
#include "lwip/stats.h"
#include "lwip/def.h"
#include "lwip/mem.h"
#include "lwip/memp.h"
#include "lwip/pbuf.h"
#include "lwip/sys.h"


static u8_t pbuf_pool_memory[(PBUF_POOL_SIZE * MEM_ALIGN_SIZE(PBUF_POOL_BUFSIZE + sizeof(struct pbuf)))];

#if !SYS_LIGHTWEIGHT_PROT
static volatile u8_t pbuf_pool_free_lock, pbuf_pool_alloc_lock;
static sys_sem_t pbuf_pool_free_sem;
#endif

static struct pbuf *pbuf_pool = NULL;
static struct pbuf *pbuf_pool_alloc_cache = NULL;
static struct pbuf *pbuf_pool_free_cache = NULL;

void
pbuf_init(void)
{
  struct pbuf *p, *q = NULL;
  u16_t i;

  pbuf_pool = (struct pbuf *)&pbuf_pool_memory[0];
  LWIP_ASSERT("pbuf_init: pool aligned", (long)pbuf_pool % MEM_ALIGNMENT == 0);
   
#ifdef PBUF_STATS
  lwip_stats.pbuf.avail = PBUF_POOL_SIZE;
#endif /* PBUF_STATS */
  
  /* Set up ->next pointers to link the pbufs of the pool together */
  p = pbuf_pool;
  
  for(i = 0; i < PBUF_POOL_SIZE; ++i) {
    p->next = (struct pbuf *)((u8_t *)p + PBUF_POOL_BUFSIZE + sizeof(struct pbuf));
    p->len = p->tot_len = PBUF_POOL_BUFSIZE;
    p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf)));
    q = p;
    p = p->next;
  }
  
  /* The ->next pointer of last pbuf is NULL to indicate that there
     are no more pbufs in the pool */
  q->next = NULL;

#if !SYS_LIGHTWEIGHT_PROT  
  pbuf_pool_alloc_lock = 0;
  pbuf_pool_free_lock = 0;
  pbuf_pool_free_sem = sys_sem_new(1);
#endif  
}

static struct pbuf *
pbuf_pool_alloc(void)
{
  struct pbuf *p = NULL;

  unsigned int state;
  sys_stop_interrupts(&state);
  /* First, see if there are pbufs in the cache. */
  if(pbuf_pool_alloc_cache) {
    p = pbuf_pool_alloc_cache;
    if(p) {
      pbuf_pool_alloc_cache = p->next; 
    }
  } else {
#if !SYS_LIGHTWEIGHT_PROT      
    /* Next, check the actual pbuf pool, but if the pool is locked, we
       pretend to be out of buffers and return NULL. */
    if(pbuf_pool_free_lock) {
#ifdef PBUF_STATS
      ++lwip_stats.pbuf.alloc_locked;
#endif /* PBUF_STATS */
      return NULL;
    }
    pbuf_pool_alloc_lock = 1;
    if(!pbuf_pool_free_lock) {
#endif /* SYS_LIGHTWEIGHT_PROT */        
      p = pbuf_pool;
      if(p) {
        pbuf_pool = p->next; 
      }
#if !SYS_LIGHTWEIGHT_PROT      
#ifdef PBUF_STATS
    } else {
      ++lwip_stats.pbuf.alloc_locked;
#endif /* PBUF_STATS */
    }
    pbuf_pool_alloc_lock = 0;
#endif /* SYS_LIGHTWEIGHT_PROT */    
  }
  
#ifdef PBUF_STATS
  if(p != NULL) {    
    ++lwip_stats.pbuf.used;
    if(lwip_stats.pbuf.used > lwip_stats.pbuf.max) {
      lwip_stats.pbuf.max = lwip_stats.pbuf.used;
    }
  }
#endif /* PBUF_STATS */

  sys_allow_interrupts(&state);
  return p;   
}

static void
pbuf_pool_free(struct pbuf *p)
{
  struct pbuf *q;
  unsigned int state;
  sys_stop_interrupts(&state);

#ifdef PBUF_STATS
    for(q = p; q != NULL; q = q->next) {
      --lwip_stats.pbuf.used;
    }
#endif /* PBUF_STATS */

  if(pbuf_pool_alloc_cache == NULL) {
    pbuf_pool_alloc_cache = p;
  } else {  
    for(q = pbuf_pool_alloc_cache; q->next != NULL; q = q->next);
    q->next = p;    
  }
  sys_allow_interrupts(&state);
}

struct pbuf *
pbuf_alloc(pbuf_layer l, u16_t length, pbuf_flag flag)
{
  struct pbuf *p, *q, *r;
  u16_t offset;
  s32_t rem_len; /* remaining length */

  /* determine header offset */
  offset = 0;
  switch (l) {
  case PBUF_TRANSPORT:
    /* add room for transport (often TCP) layer header */
    offset += PBUF_TRANSPORT_HLEN;
    /* FALLTHROUGH */
  case PBUF_IP:
    /* add room for IP layer header */
    offset += PBUF_IP_HLEN;
    /* FALLTHROUGH */
  case PBUF_LINK:
    /* add room for link layer header */
    offset += PBUF_LINK_HLEN;
    break;
  case PBUF_RAW:
    break;
  default:
    LWIP_ASSERT("pbuf_alloc: bad pbuf layer", 0);
    return NULL;
  }

  switch (flag) {
  case PBUF_POOL:
    /* allocate head of pbuf chain into p */
    p = pbuf_pool_alloc();
    if (p == NULL) {
#ifdef PBUF_STATS
      ++lwip_stats.pbuf.err;
#endif /* PBUF_STATS */
      return NULL;
    }
    p->next = NULL;
    
    /* make the payload pointer point 'offset' bytes into pbuf data memory */
    p->payload = MEM_ALIGN((void *)((u8_t *)p + (sizeof(struct pbuf) + offset)));
    LWIP_ASSERT("pbuf_alloc: pbuf p->payload properly aligned",
            ((u32_t)p->payload % MEM_ALIGNMENT) == 0);
    /* the total length of the pbuf chain is the requested size */
    p->tot_len = length;
    /* set the length of the first pbuf in the chain */
    p->len = length > PBUF_POOL_BUFSIZE - offset? PBUF_POOL_BUFSIZE - offset: length;
    /* set pbuf type */
    p->flags = PBUF_FLAG_POOL;
    
    /* now allocate the tail of the pbuf chain */
    
    /* remember first pbuf for linkage in next iteration */
    r = p;
    /* remaining length to be allocated */
    rem_len = length - p->len;
    /* any remaining pbufs to be allocated? */
    while(rem_len > 0) {      
      q = pbuf_pool_alloc();
      if (q == NULL) {
       DEBUGF(PBUF_DEBUG | 2, ("pbuf_alloc: Out of pbufs in pool.\n"));
#ifdef PBUF_STATS
        ++lwip_stats.pbuf.err;
#endif /* PBUF_STATS */
        /* bail out unsuccesfully */
        pbuf_pool_free(p);
        return NULL;
      }
      //q->next = NULL;
      /* make previous pbuf point to this pbuf */
      r->next = q;
      /* set length of this pbuf */
      q->len = rem_len > PBUF_POOL_BUFSIZE? PBUF_POOL_BUFSIZE: rem_len;
      q->flags = PBUF_FLAG_POOL;
      q->payload = (void *)((u8_t *)q + sizeof(struct pbuf));
      LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
              ((u32_t)q->payload % MEM_ALIGNMENT) == 0);
      q->ref = 1;
      /* calculate remaining length to be allocated */
      rem_len -= q->len;
      /* remember this pbuf for linkage in next iteration */
      r = q;
    }
    /* end of chain */
    r->next = NULL;

    break;
  case PBUF_RAM:
    /* If pbuf is to be allocated in RAM, allocate memory for it. */
    p = mem_malloc(MEM_ALIGN_SIZE(sizeof(struct pbuf) + length + offset));
    if (p == NULL) {
      return NULL;
    }
    /* Set up internal structure of the pbuf. */
    p->payload = MEM_ALIGN((void *)((u8_t *)p + sizeof(struct pbuf) + offset));
    p->len = p->tot_len = length;
    p->next = NULL;
    p->flags = PBUF_FLAG_RAM;

    LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
           ((u32_t)p->payload % MEM_ALIGNMENT) == 0);
    break;
  /* pbuf references existing (static constant) ROM payload? */
  case PBUF_ROM:
  /* pbuf references existing (externally allocated) RAM payload? */
  case PBUF_REF:
    /* only allocate memory for the pbuf structure */
    p = memp_mallocp(MEMP_PBUF);
    if (p == NULL) {
      DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_alloc: Could not allocate MEMP_PBUF for PBUF_REF.\n"));
      return NULL;
    }
    /* caller must set this field properly, afterwards */
    p->payload = NULL;
    p->len = p->tot_len = length;
    p->next = NULL;
    p->flags = (flag == PBUF_ROM? PBUF_FLAG_ROM: PBUF_FLAG_REF);
    break;
  default:
    LWIP_ASSERT("pbuf_alloc: erroneous flag", 0);
    return NULL;
  }
  p->ref = 1;
  return p;
}

void
pbuf_refresh(void)
{
  struct pbuf *p;
  unsigned int state;
  sys_stop_interrupts(&state);
 
#if !SYS_LIGHTWEIGHT_PROT
  sys_sem_wait(pbuf_pool_free_sem);
#endif /* else SYS_LIGHTWEIGHT_PROT */
  
  if(pbuf_pool_free_cache != NULL) {
#if !SYS_LIGHTWEIGHT_PROT      
    pbuf_pool_free_lock = 1;
    if(!pbuf_pool_alloc_lock) {
#endif /* SYS_LIGHTWEIGHT_PROT */
      if(pbuf_pool == NULL) {
        pbuf_pool = pbuf_pool_free_cache;       
      } else {  
        for(p = pbuf_pool; p->next != NULL; p = p->next);
        p->next = pbuf_pool_free_cache;   
      }
      pbuf_pool_free_cache = NULL;
#if !SYS_LIGHTWEIGHT_PROT      
#ifdef PBUF_STATS
    } else {
      ++lwip_stats.pbuf.refresh_locked;
#endif /* PBUF_STATS */
    }
    
    pbuf_pool_free_lock = 0;
#endif /* SYS_LIGHTWEIGHT_PROT */    
  }
  sys_allow_interrupts(&state);
#if !SYS_LIGHTWEIGHT_PROT      
  sys_sem_signal(pbuf_pool_free_sem);
#endif /* SYS_LIGHTWEIGHT_PROT */  
}

#ifdef PBUF_STATS
#define DEC_PBUF_STATS do { --lwip_stats.pbuf.used; } while (0)
#else /* PBUF_STATS */
#define DEC_PBUF_STATS
#endif /* PBUF_STATS */

#define PBUF_POOL_FAST_FREE(p)  do {                                    \
                                  p->next = pbuf_pool_free_cache;       \
                                  pbuf_pool_free_cache = p;             \
                                  DEC_PBUF_STATS;                       \
                                } while (0)

#if SYS_LIGHTWEIGHT_PROT
#define PBUF_POOL_FREE(p)  do {                                         \
                                unsigned int state;                     \
                                sys_stop_interrupts(&state);            \
                                PBUF_POOL_FAST_FREE(p);                 \
                                sys_allow_interrupts(&state)            \
                               } while(0)
#else /* SYS_LIGHTWEIGHT_PROT */
#define PBUF_POOL_FREE(p)  do {                                         \
                             sys_sem_wait(pbuf_pool_free_sem);          \
                             PBUF_POOL_FAST_FREE(p);                    \
                             sys_sem_signal(pbuf_pool_free_sem);        \
                           } while(0)
#endif /* SYS_LIGHTWEIGHT_PROT */

void
pbuf_realloc(struct pbuf *p, u16_t new_len)
{
  struct pbuf *q;
  u16_t rem_len; /* remaining length */
  s16_t grow;

  LWIP_ASSERT("pbuf_realloc: sane p->flags", p->flags == PBUF_FLAG_POOL ||
              p->flags == PBUF_FLAG_ROM ||
              p->flags == PBUF_FLAG_RAM ||
              p->flags == PBUF_FLAG_REF);

  /* desired length larger than current length? */
  if (new_len >= p->tot_len) {
    /* enlarging not yet supported */
    return;
  }
  
  /* the pbuf chain grows by (new_len - p->tot_len) bytes
   * (which may be negative in case of shrinking) */
  grow = new_len - p->tot_len;
  
  /* first, step over any pbufs that should remain in the chain */
  rem_len = new_len;
  q = p;  
  /* this pbuf should be kept? */
  while (rem_len > q->len) {
    /* decrease remaining length by pbuf length */
    rem_len -= q->len;
    /* decrease total length indicator */
    q->tot_len += grow;
    /* proceed to next pbuf in chain */
    q = q->next;
  }
  /* we have now reached the new last pbuf (in q) */
  /* rem_len == desired length for pbuf q */  

  /* shrink allocated memory for PBUF_RAM */
  /* (other types merely adjust their length fields */
  if ((q->flags == PBUF_FLAG_RAM) && (rem_len != q->len)) {
    /* reallocate and adjust the length of the pbuf that will be split */
    mem_realloc(q, (u8_t *)q->payload - (u8_t *)q + rem_len);
  }
  /* adjust length fields for new last pbuf */
  q->len = rem_len;
  q->tot_len = q->len;

  /* any remaining pbufs in chain? */
  if (q->next != NULL) {
    /* free remaining pbufs in chain */
    pbuf_free(q->next);
  }
  /* q is last packet in chain */
  q->next = NULL;

  pbuf_refresh();
}

u8_t
pbuf_header(struct pbuf *p, s16_t header_size)
{
  void *payload;

  /* remember current payload pointer */
  payload = p->payload;

  /* pbuf types containing payloads? */
  if (p->flags == PBUF_FLAG_RAM || p->flags == PBUF_FLAG_POOL) {
    /* set new payload pointer */
    p->payload = (u8_t *)p->payload - header_size;
    /* boundary check fails? */
    if ((u8_t *)p->payload < (u8_t *)p + sizeof(struct pbuf)) {
      DEBUGF( PBUF_DEBUG | 2, ("pbuf_header: failed as %p < %p\n",
        (u8_t *)p->payload,
        (u8_t *)p + sizeof(struct pbuf)) );\
      /* restore old payload pointer */
      p->payload = payload;
      /* bail out unsuccesfully */
      return 1;
    }
  /* pbuf types refering to payloads? */
  } else if (p->flags == PBUF_FLAG_REF || p->flags == PBUF_FLAG_ROM) {
    /* hide a header in the payload? */
    if ((header_size < 0) && (header_size - p->len <= 0)) {
      /* increase payload pointer */
      p->payload = (u8_t *)p->payload - header_size;
    } else {
      /* cannot expand payload to front (yet!)
       * bail out unsuccesfully */
      return 1;
    }
  }
  DEBUGF( PBUF_DEBUG, ("pbuf_header: old %p new %p (%d)\n", payload, p->payload, header_size) );
  /* modify pbuf length fields */
  p->len += header_size;
  p->tot_len += header_size;

  return 0;
}

u8_t
pbuf_free(struct pbuf *p)
{
  struct pbuf *q;
  u8_t count;
  unsigned int state;

  if (p == NULL) {
    DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_free(p == NULL) was called.\n"));
    return 0;
  }

//  PERF_START;

  LWIP_ASSERT("pbuf_free: sane flags",
    p->flags == PBUF_FLAG_RAM || p->flags == PBUF_FLAG_ROM ||
    p->flags == PBUF_FLAG_REF || p->flags == PBUF_FLAG_POOL);

  count = 0;
  /* Since decrementing ref cannot be guaranteed to be a single machine operation
   * we must protect it. Also, the later test of ref must be protected.
   */
  sys_stop_interrupts(&state);
  /* de-allocate all consecutive pbufs from the head of the chain that
   * obtain a zero reference count */
  while (p != NULL) {
    /* all pbufs in a chain are referenced at least once */
    LWIP_ASSERT("pbuf_free: p->ref > 0", p->ref > 0);
    p->ref--;
    /* this pbuf is no longer referenced to? */
    if (p->ref == 0) {
      /* remember next pbuf in chain for next iteration */
      q = p->next;
      /* is this a pbuf from the pool? */
      if (p->flags == PBUF_FLAG_POOL) {
        p->len = p->tot_len = PBUF_POOL_BUFSIZE;
        p->payload = (void *)((u8_t *)p + sizeof(struct pbuf));
        PBUF_POOL_FREE(p);
      /* a RAM/ROM referencing pbuf */
      } else if (p->flags == PBUF_FLAG_ROM || p->flags == PBUF_FLAG_REF) {
        memp_freep(MEMP_PBUF, p);
      /* pbuf with data */
      } else {
        mem_free(p);
      }
      count++;
      /* proceed to next pbuf */
      p = q;
    /* p->ref > 0, this pbuf is still referenced to */
    /* (so the remaining pbufs in chain as well)    */
    } else {
      /* stop walking through chain */
      p = NULL;
    }
  }
  sys_allow_interrupts(&state);
  pbuf_refresh();
//  PERF_STOP("pbuf_free");
  /* return number of de-allocated pbufs */
  return count;
}

u8_t
pbuf_clen(struct pbuf *p)
{
  u8_t len;

  len = 0;  
  while (p != NULL) {
    ++len;
    p = p->next;
  }
  return len;
}
void
pbuf_ref(struct pbuf *p)
{
  unsigned int state;
  /* pbuf given? */  
  if (p != NULL) {
    sys_stop_interrupts(&state);
    ++(p->ref);
    sys_allow_interrupts(&state);
  }
}

void
pbuf_ref_chain(struct pbuf *p)
{
  unsigned int state;
  sys_stop_interrupts(&state);
    
  while (p != NULL) {
    ++p->ref;
    p = p->next;
  }
  sys_allow_interrupts(&state);
}


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

  LWIP_ASSERT("h != NULL", h != NULL);
  LWIP_ASSERT("t != NULL", t != NULL);
  
  /* 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 */
  /* add total length of second chain to last pbuf total of first chain */
  p->tot_len += t->tot_len;
  /* chain last pbuf of h chain (p) with first of tail (t) */
  p->next = t;
  /* t is now referenced to one more time */
  pbuf_ref(t);
  DEBUGF(PBUF_DEBUG | DBG_FRESH | 2, ("pbuf_chain: referencing tail %p\n", (void *) t));
}

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 */
    DEBUGF(PBUF_DEBUG | DBG_STATE, ("pbuf_dechain: unreferencing %p\n", (void *) q));
    tail_gone = pbuf_free(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);
}

struct pbuf *
pbuf_take(struct pbuf *p)
{
  struct pbuf *q , *prev, *head;
  LWIP_ASSERT("pbuf_take: p != NULL\n", p != NULL);
  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) {
      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) DEBUGF(PBUF_DEBUG | DBG_TRACE | 2, ("pbuf_take: Could not allocate PBUF_POOL\n"));
      } else {
        /* no replacement pbuf yet */
        q = NULL;
        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) 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 */
        DEBUGF(PBUF_DEBUG, ("pbuf_take: replaced PBUF_REF %p with %p\n", (void *)p, (void *)q));
        p = q;
      } else {
        /* deallocate chain */
        pbuf_free(head);
        DEBUGF(PBUF_DEBUG | 2, ("pbuf_take: failed to allocate replacement pbuf for %p\n", (void *)p));
        return NULL;
      }
    /* p->flags != PBUF_FLAG_REF */
    } else {
      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);
  DEBUGF(PBUF_DEBUG | DBG_TRACE | 1, ("pbuf_take: end of chain reached.\n"));
  
  return head;
}

---