text: /**
   * @file
   * Packet buffer management
   *
   * Packets are built from the pbuf data structure. It supports dynamic
   * memory allocation for packet contents or can reference externally
   * managed packet contents both in RAM and ROM. Quick allocation for
   * incoming packets is provided through pools with fixed sized pbufs.
   *
   * A packet may span over multiple pbufs, chained as a singly linked
   * list. This is called a "pbuf chain".
   *
   * Multiple packets may be queued, also using this singly linked list.
   * This is called a "packet queue".
   * 
   * So, a packet queue consists of one or more pbuf chains, each of
   * which consist of one or more pbufs. CURRENTLY, PACKET QUEUES ARE
   * NOT SUPPORTED!!! Use helper structs to queue multiple packets.
   * 
   * The differences between a pbuf chain and a packet queue are very
   * precise but subtle. 
   *
   * The last pbuf of a packet has a ->tot_len field that equals the
   * ->len field. It can be found by traversing the list. If the last
   * pbuf of a packet has a ->next field other than NULL, more packets
   * are on the queue.
   *
   * Therefore, looping through a pbuf of a single packet, has an
   * loop end condition (tot_len == p->len), NOT (next == NULL).
   */
  
  /*
   * Copyright (c) 2001-2004 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>
   *
   */
  
  #include "lwip/opt.h"
  
  #include "lwip/stats.h"
  #include "lwip/def.h"
  #include "lwip/mem.h"
  #include "lwip/memp.h"
  #include "lwip/lwip_pbuf.h"
  #include "lwip/sys.h"
  #include "arch/perf.h"
  #if LWIP_TCP && TCP_QUEUE_OOSEQ
  #include "lwip/tcp_impl.h"
  #endif
  #if LWIP_CHECKSUM_ON_COPY
  #include "lwip/inet_chksum.h"
  #endif
  
  #include <string.h>
  
  #define PBUF_SAVE 1
  
  /* Since the pool is created in memp, PBUF_POOL_BUFSIZE will be automatically
     aligned there. Therefore, PBUF_POOL_BUFSIZE_ALIGNED can be used here. */
  #define PBUF_POOL_BUFSIZE_ALIGNED LWIP_MEM_ALIGN_SIZE(PBUF_POOL_BUFSIZE)
  
  #if !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ
  #define PBUF_POOL_IS_EMPTY()
  #else /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ */
  
  #if !NO_SYS
  #ifndef PBUF_POOL_FREE_OOSEQ_QUEUE_CALL
  #include "lwip/tcpip.h"
  #define PBUF_POOL_FREE_OOSEQ_QUEUE_CALL()  do { \
    if(tcpip_callback_with_block(pbuf_free_ooseq_callback, NULL, 0) != ERR_OK) { \
        SYS_ARCH_PROTECT(old_level); \
        pbuf_free_ooseq_pending = 0; \
        SYS_ARCH_UNPROTECT(old_level); \
    } } while(0)
  #endif /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
  #endif /* !NO_SYS */
  
  volatile u8_t pbuf_free_ooseq_pending;
  #define PBUF_POOL_IS_EMPTY() pbuf_pool_is_empty()
  
  /**
   * Attempt to reclaim some memory from queued out-of-sequence TCP segments
   * if we run out of pool pbufs. It's better to give priority to new packets
   * if we're running out.
   *
   * This must be done in the correct thread context therefore this function
   * can only be used with NO_SYS=0 and through tcpip_callback.
   */
  #if !NO_SYS
  static
  #endif /* !NO_SYS */
  void
  pbuf_free_ooseq(void)
  {
    struct tcp_pcb* pcb;
    SYS_ARCH_DECL_PROTECT(old_level);
  
    SYS_ARCH_PROTECT(old_level);
    pbuf_free_ooseq_pending = 0;
    SYS_ARCH_UNPROTECT(old_level);
  
    for (pcb = tcp_active_pcbs; NULL != pcb; pcb = pcb->next) {
      if (NULL != pcb->ooseq) {
        /** Free the ooseq pbufs of one PCB only */
        LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free_ooseq: freeing out-of-sequence pbufs\r\n"));
        tcp_segs_free(pcb->ooseq);
        pcb->ooseq = NULL;
        return;
      }
    }
  }
  
  #if !NO_SYS
  /**
   * Just a callback function for tcpip_timeout() that calls pbuf_free_ooseq().
   */
  static void
  pbuf_free_ooseq_callback(void *arg)
  {
    LWIP_UNUSED_ARG(arg);
    pbuf_free_ooseq();
  }
  #endif /* !NO_SYS */
  
  /** Queue a call to pbuf_free_ooseq if not already queued. */
  static void
  pbuf_pool_is_empty(void)
  {
  #ifndef PBUF_POOL_FREE_OOSEQ_QUEUE_CALL
    SYS_ARCH_DECL_PROTECT(old_level);
    SYS_ARCH_PROTECT(old_level);
    pbuf_free_ooseq_pending = 1;
    SYS_ARCH_UNPROTECT(old_level);
  #else /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
    u8_t queued;
    SYS_ARCH_DECL_PROTECT(old_level);
    SYS_ARCH_PROTECT(old_level);
    queued = pbuf_free_ooseq_pending;
    pbuf_free_ooseq_pending = 1;
    SYS_ARCH_UNPROTECT(old_level);
  
    if(!queued) {
      /* queue a call to pbuf_free_ooseq if not already queued */
      PBUF_POOL_FREE_OOSEQ_QUEUE_CALL();
    }
  #endif /* PBUF_POOL_FREE_OOSEQ_QUEUE_CALL */
  }
  #endif /* !LWIP_TCP || !TCP_QUEUE_OOSEQ || !PBUF_POOL_FREE_OOSEQ */
  
  /**
   * Allocates a pbuf of the given type (possibly a chain for PBUF_POOL type).
   *
   * The actual memory allocated for the pbuf is determined by the
   * layer at which the pbuf is allocated and the requested size
   * (from the size parameter).
   *
   * @param layer flag to define header size
   * @param length size of the pbuf's payload
   * @param type this parameter decides how and where the pbuf
   * should be allocated as follows:
   *
   * - PBUF_RAM: buffer memory for pbuf is allocated as one large
   *             chunk. This includes protocol headers as well.
   * - PBUF_ROM: no buffer memory is allocated for the pbuf, even for
   *             protocol headers. Additional headers must be prepended
   *             by allocating another pbuf and chain in to the front of
   *             the ROM pbuf. It is assumed that the memory used is really
   *             similar to ROM in that it is immutable and will not be
   *             changed. Memory which is dynamic should generally not
   *             be attached to PBUF_ROM pbufs. Use PBUF_REF instead.
   * - PBUF_REF: no buffer memory is allocated for the pbuf, even for
   *             protocol headers. It is assumed that the pbuf is only
   *             being used in a single thread. If the pbuf gets queued,
   *             then pbuf_take should be called to copy the buffer.
   * - PBUF_POOL: the pbuf is allocated as a pbuf chain, with pbufs from
   *              the pbuf pool that is allocated during pbuf_init().
   *
   * @return the allocated pbuf. If multiple pbufs where allocated, this
   * is the first pbuf of a pbuf chain.
   */
  #if CONFIG_MEMP_DEBUG
  
  const char * const tcp_dbg_str[] = {
        "NONE",                       //PBUF_DBG_FLAG_RESET
        "TCP",                        //PBUF_DBG_FLAG_TCP
        "UDP",                        //PBUF_DBG_FLAG_UDP
        "IP",                         //PBUF_DBG_FLAG_IP
        "ARP",                        //PBUF_DBG_FLAG_ARP
        "L2",                         //PBUF_DBG_FLAG_L2
        "L2_TX_DRV",                  //PBUF_DBG_FLAG_L2_TX_DRIVER
        "L2_CMDENG",                  //PBUF_DBG_FLAG_L2_CMDENG
      };
  
  struct pbuf *
  pbuf_alloc_fn(pbuf_layer layer, u16_t length, pbuf_type type, const char* file, const int line)
  #else//#if CONFIG_MEMP_DEBUG
  struct pbuf *
  pbuf_alloc(pbuf_layer layer, u16_t length, pbuf_type type)
  #endif//#if CONFIG_MEMP_DEBUG
  {
    struct pbuf *p, *q, *r;
    u16_t offset;
    s32_t rem_len; /* remaining length */
  #if PBUF_SAVE
    u16_t reqlen;
  #endif
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F")\r\n", length));
  
    /* determine header offset */
    offset = 0;
    switch (layer) {
    case PBUF_TRANSPORT:
      /* add room for transport (often TCP) layer header */
      offset += PBUF_LINK_HLEN + PBUF_IP_HLEN + PBUF_TRANSPORT_HLEN;
      break;
    case PBUF_IP:
      /* add room for IP layer header */
      offset += PBUF_LINK_HLEN + PBUF_IP_HLEN;
      break;
    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 (type) {
    case PBUF_POOL:
    case PBUF_RAM:
          
      
      /* allocate head of pbuf chain into p */
  #if PBUF_SAVE
      reqlen = LWIP_MIN(LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF + offset + length), PBUF_POOL_BUFSIZE);
      p = (struct pbuf *)OS_MemAlloc(reqlen);
  #else
      p = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL);
  #endif
      LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc: allocated pbuf %p\r\n", (void *)p));
      if (p == NULL) {
        PBUF_POOL_IS_EMPTY();
        return NULL;
      }
  
  #if CONFIG_MEMP_DEBUG
      p->line = line;
      p->file = file;
      p->dbg_flags = 0;
  #endif
  
          
      p->type = PBUF_POOL;
      p->next = NULL;
  
      /* make the payload pointer point 'offset' bytes into pbuf data memory */
      p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p + (SIZEOF_STRUCT_PBUF + offset)));
      LWIP_ASSERT("pbuf_alloc: pbuf p->payload properly aligned",
              ((mem_ptr_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 */
  #if PBUF_SAVE
      p->len = LWIP_MIN(length, reqlen - LWIP_MEM_ALIGN_SIZE(offset + SIZEOF_STRUCT_PBUF));
  #else
      p->len = LWIP_MIN(length, PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset + SIZEOF_STRUCT_PBUF));
  #endif
      LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
                  ((u8_t*)p->payload + p->len <=
                   (u8_t*)p + PBUF_POOL_BUFSIZE_ALIGNED));
      LWIP_ASSERT("PBUF_POOL_BUFSIZE must be bigger than MEM_ALIGNMENT",
        (PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)) > 0 );
  
  
  
      /* set reference count (needed here in case we fail) */
      p->ref = 1;
  
      /* 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;
  
  	
  	//just can allocated tot_len equeal to p->len
  	#if 0
  	if(p->len != p->tot_len){
  		LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("PBUF_POOL_BUFSIZE must be bigger than MEM_ALIGNMENT p->len[%d]  p->tot_len[%d]\r\n", p->len,  p->tot_len));
  		pbuf_free(p);
          return NULL;		
  	}
      #endif		
      
      /* any remaining pbufs to be allocated? */
      while (rem_len > 0) {
  #if PBUF_SAVE
        reqlen = LWIP_MIN(LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF + rem_len), PBUF_POOL_BUFSIZE);
        q = (struct pbuf *)OS_MemAlloc(reqlen);
  #else
        q = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL);
  #endif
        if (q == NULL) {
          PBUF_POOL_IS_EMPTY();
          /* free chain so far allocated */
          pbuf_free(p);
          /* bail out unsuccesfully */
          return NULL;
        }
        q->type = PBUF_POOL/*type*/;
        q->flags = 0;
        q->next = NULL;
        /* make previous pbuf point to this pbuf */
        r->next = q;
        /* set total length of this pbuf and next in chain */
        LWIP_ASSERT("rem_len < max_u16_t", rem_len < 0xffff);
        q->tot_len = (u16_t)rem_len;
        /* this pbuf length is pool size, unless smaller sized tail */
  #if PBUF_SAVE
        q->len = LWIP_MIN((u16_t)rem_len, reqlen - LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF));
  #else
        q->len = LWIP_MIN((u16_t)rem_len, PBUF_POOL_BUFSIZE_ALIGNED);
  #endif
        q->payload = (void *)((u8_t *)q + SIZEOF_STRUCT_PBUF);
        LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
                ((mem_ptr_t)q->payload % MEM_ALIGNMENT) == 0);
        LWIP_ASSERT("check p->payload + p->len does not overflow pbuf",
                    ((u8_t*)p->payload + p->len <=
                     (u8_t*)p + SIZEOF_STRUCT_PBUF + PBUF_POOL_BUFSIZE_ALIGNED));
        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;
  
  #if 0
    case PBUF_RAM:
      /* If pbuf is to be allocated in RAM, allocate memory for it. */
      p = (struct pbuf*)mem_malloc(LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF + offset) + LWIP_MEM_ALIGN_SIZE(length));
      if (p == NULL) {
        return NULL;
      }
      /* Set up internal structure of the pbuf. */
      p->payload = LWIP_MEM_ALIGN((void *)((u8_t *)p + SIZEOF_STRUCT_PBUF + offset));
      p->len = p->tot_len = length;
      p->next = NULL;
      p->type = type;
  
      LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
             ((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
      break;
  #endif
  
    /* pbuf references existing (non-volatile static constant) ROM payload? */
    case PBUF_ROM:
    /* pbuf references existing (externally allocated) RAM payload? */
    case PBUF_REF:
      /* only allocate memory for the pbuf structure */
  
  //    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("pbuf allocate different type[%d]\r\n", type));
  
  
      //Just allocate pbuf header    
      p = (struct pbuf *)memp_malloc(MEMP_PBUF);
      if (p == NULL) {
        LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
                    ("pbuf_alloc: Could not allocate MEMP_PBUF for PBUF_%s.\r\n",
                    (type == PBUF_ROM) ? "ROM" : "REF"));
        return NULL;
      }
      /* caller must set this field properly, afterwards */
      p->payload = NULL;
      p->len = p->tot_len = length;
      p->next = NULL;
      p->type = type;
      break;
    default:
      LWIP_ASSERT("pbuf_alloc: erroneous type", 0);
      return NULL;
    }
    /* set reference count */
    p->ref = 1;
    /* set flags */
    p->flags = 0;
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F") == %p\r\n", length, (void *)p));
    return p;
  }
  
  #if LWIP_SUPPORT_CUSTOM_PBUF
  /** Initialize a custom pbuf (already allocated).
   *
   * @param layer flag to define header size
   * @param length size of the pbuf's payload
   * @param type type of the pbuf (only used to treat the pbuf accordingly, as
   *        this function allocates no memory)
   * @param p pointer to the custom pbuf to initialize (already allocated)
   * @param payload_mem pointer to the buffer that is used for payload and headers,
   *        must be at least big enough to hold 'length' plus the header size,
   *        may be NULL if set later.
   *        ATTENTION: The caller is responsible for correct alignment of this buffer!!
   * @param payload_mem_len the size of the 'payload_mem' buffer, must be at least
   *        big enough to hold 'length' plus the header size
   */
  struct pbuf*
  pbuf_alloced_custom(pbuf_layer l, u16_t length, pbuf_type type, struct pbuf_custom *p,
                      void *payload_mem, u16_t payload_mem_len)
  {
    u16_t offset;
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloced_custom(length=%"U16_F")\r\n", length));
  
    /* determine header offset */
    offset = 0;
    switch (l) {
    case PBUF_TRANSPORT:
      /* add room for transport (often TCP) layer header */
      offset += PBUF_LINK_HLEN + PBUF_IP_HLEN + PBUF_TRANSPORT_HLEN;
      break;
    case PBUF_IP:
      /* add room for IP layer header */
      offset += PBUF_LINK_HLEN + PBUF_IP_HLEN;
      break;
    case PBUF_LINK:
      /* add room for link layer header */
      offset += PBUF_LINK_HLEN;
      break;
    case PBUF_RAW:
      break;
    default:
      LWIP_ASSERT("pbuf_alloced_custom: bad pbuf layer", 0);
      return NULL;
    }
  
    if (LWIP_MEM_ALIGN_SIZE(offset) + length < payload_mem_len) {
      LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_WARNING, ("pbuf_alloced_custom(length=%"U16_F") buffer too short\r\n", length));
      return NULL;
    }
  
    p->pbuf.next = NULL;
    if (payload_mem != NULL) {
      p->pbuf.payload = (u8_t *)payload_mem + LWIP_MEM_ALIGN_SIZE(offset);
    } else {
      p->pbuf.payload = NULL;
    }
    p->pbuf.flags = PBUF_FLAG_IS_CUSTOM;
    p->pbuf.len = p->pbuf.tot_len = length;
    p->pbuf.type = type;
    p->pbuf.ref = 1;
    return &p->pbuf;
  }
  #endif /* LWIP_SUPPORT_CUSTOM_PBUF */
  
  /**
   * Shrink a pbuf chain to a desired length.
   *
   * @param p pbuf to shrink.
   * @param new_len desired new length of pbuf chain
   *
   * Depending on the desired length, the first few pbufs in a chain might
   * be skipped and left unchanged. The new last pbuf in the chain will be
   * resized, and any remaining pbufs will be freed.
   *
   * @note If the pbuf is ROM/REF, only the ->tot_len and ->len fields are adjusted.
   * @note May not be called on a packet queue.
   *
   * @note Despite its name, pbuf_realloc cannot grow the size of a pbuf (chain).
   */
  void
  pbuf_realloc(struct pbuf *p, u16_t new_len)
  {
    struct pbuf *q;
    u16_t rem_len; /* remaining length */
    s32_t grow;
  
    LWIP_ASSERT("pbuf_realloc: p != NULL", p != NULL);
    LWIP_ASSERT("pbuf_realloc: sane p->type", p->type == PBUF_POOL ||
                p->type == PBUF_ROM ||
                p->type == PBUF_RAM ||
                p->type == PBUF_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;
    /* should this pbuf be kept? */
    while (rem_len > q->len) {
      /* decrease remaining length by pbuf length */
      rem_len -= q->len;
      /* decrease total length indicator */
      LWIP_ASSERT("grow < max_u16_t", grow < 0xffff);
      q->tot_len += (u16_t)grow;
      /* proceed to next pbuf in chain */
      q = q->next;
      LWIP_ASSERT("pbuf_realloc: q != NULL", q != NULL);
    }
    /* 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->type == PBUF_RAM) && (rem_len != q->len)) {
      /* reallocate and adjust the length of the pbuf that will be split */
      q = (struct pbuf *)mem_trim((void*)q, (u16_t)((u8_t *)q->payload - (u8_t *)q) + rem_len);
      LWIP_ASSERT("mem_trim returned q == NULL", q != NULL);
    }
    /* 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;
  
  }
  
  /**
   * Adjusts the payload pointer to hide or reveal headers in the payload.
   *
   * Adjusts the ->payload pointer so that space for a header
   * (dis)appears in the pbuf payload.
   *
   * The ->payload, ->tot_len and ->len fields are adjusted.
   *
   * @param p pbuf to change the header size.
   * @param header_size_increment Number of bytes to increment header size which
   * increases the size of the pbuf. New space is on the front.
   * (Using a negative value decreases the header size.)
   * If hdr_size_inc is 0, this function does nothing and returns succesful.
   *
   * PBUF_ROM and PBUF_REF type buffers cannot have their sizes increased, so
   * the call will fail. A check is made that the increase in header size does
   * not move the payload pointer in front of the start of the buffer.
   * @return non-zero on failure, zero on success.
   *
   */
  u8_t
  pbuf_header(struct pbuf *p, s16_t header_size_increment)
  {
    u16_t type;
    void *payload;
    u16_t increment_magnitude;
  
    LWIP_ASSERT("p != NULL", p != NULL);
    if ((header_size_increment == 0) || (p == NULL)) {
      return 0;
    }
   
    if (header_size_increment < 0){
      increment_magnitude = -header_size_increment;
      /* Check that we aren't going to move off the end of the pbuf */
      LWIP_ERROR("increment_magnitude <= p->len", (increment_magnitude <= p->len), return 1;);
    } else {
      increment_magnitude = header_size_increment;
  #if 0
      /* Can't assert these as some callers speculatively call
           pbuf_header() to see if it's OK.  Will return 1 below instead. */
      /* Check that we've got the correct type of pbuf to work with */
      LWIP_ASSERT("p->type == PBUF_RAM || p->type == PBUF_POOL", 
                  p->type == PBUF_RAM || p->type == PBUF_POOL);
      /* Check that we aren't going to move off the beginning of the pbuf */
      LWIP_ASSERT("p->payload - increment_magnitude >= p + SIZEOF_STRUCT_PBUF",
                  (u8_t *)p->payload - increment_magnitude >= (u8_t *)p + SIZEOF_STRUCT_PBUF);
  #endif
    }
  
    type = p->type;
    /* remember current payload pointer */
    payload = p->payload;
  
    /* pbuf types containing payloads? */
    if (type == PBUF_RAM || type == PBUF_POOL) {
      /* set new payload pointer */
      p->payload = (u8_t *)p->payload - header_size_increment;
      /* boundary check fails? */
      if ((u8_t *)p->payload < (u8_t *)p + SIZEOF_STRUCT_PBUF) {
        LWIP_DEBUGF( PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
          ("pbuf_header: failed as %p < %p (not enough space for new header size)\r\n",
          (void *)p->payload, (void *)(p + 1)));
        /* restore old payload pointer */
        p->payload = payload;
        /* bail out unsuccesfully */
        return 1;
      }
    /* pbuf types refering to external payloads? */
    } else if (type == PBUF_REF || type == PBUF_ROM) {
      /* hide a header in the payload? */
      if ((header_size_increment < 0) && (increment_magnitude <= p->len)) {
        /* increase payload pointer */
        p->payload = (u8_t *)p->payload - header_size_increment;
      } else {
        /* cannot expand payload to front (yet!)
         * bail out unsuccesfully */
        return 1;
      }
    } else {
      /* Unknown type */
      LWIP_ASSERT("bad pbuf type", 0);
      return 1;
    }
    /* modify pbuf length fields */
    p->len += header_size_increment;
    p->tot_len += header_size_increment;
  
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_header: old %p new %p (%"S16_F")\r\n",
      (void *)payload, (void *)p->payload, header_size_increment));
  
    return 0;
  }
  
  /**
   * Dereference a pbuf chain or queue and deallocate any no-longer-used
   * pbufs at the head of this chain or queue.
   *
   * Decrements the pbuf reference count. If it reaches zero, the pbuf is
   * deallocated.
   *
   * For a pbuf chain, this is repeated for each pbuf in the chain,
   * up to the first pbuf which has a non-zero reference count after
   * decrementing. So, when all reference counts are one, the whole
   * chain is free'd.
   *
   * @param p The pbuf (chain) to be dereferenced.
   *
   * @return the number of pbufs that were de-allocated
   * from the head of the chain.
   *
   * @note MUST NOT be called on a packet queue (Not verified to work yet).
   * @note the reference counter of a pbuf equals the number of pointers
   * that refer to the pbuf (or into the pbuf).
   *
   * @internal examples:
   *
   * Assuming existing chains a->b->c with the following reference
   * counts, calling pbuf_free(a) results in:
   * 
   * 1->2->3 becomes ...1->3
   * 3->3->3 becomes 2->3->3
   * 1->1->2 becomes ......1
   * 2->1->1 becomes 1->1->1
   * 1->1->1 becomes .......
   *
   */
  u8_t
  pbuf_free(struct pbuf *p)
  {
    u16_t type;
    struct pbuf *q;
    u8_t count;
  
    if (p == NULL) {
      LWIP_ASSERT("p != NULL", p != NULL);
      /* if assertions are disabled, proceed with debug output */
      LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_LEVEL_SERIOUS,
        ("pbuf_free(p == NULL) was called.\r\n"));
      return 0;
    }
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free(%p)\r\n", (void *)p));
  
    PERF_START;
  
    LWIP_ASSERT("pbuf_free: sane type",
      p->type == PBUF_RAM || p->type == PBUF_ROM ||
      p->type == PBUF_REF || p->type == PBUF_POOL);
  
    count = 0;
    /* de-allocate all consecutive pbufs from the head of the chain that
     * obtain a zero reference count after decrementing*/
    while (p != NULL) {
      u16_t ref;
      SYS_ARCH_DECL_PROTECT(old_level);
      /* Since decrementing ref cannot be guaranteed to be a single machine operation
       * we must protect it. We put the new ref into a local variable to prevent
       * further protection. */
      SYS_ARCH_PROTECT(old_level);
      /* all pbufs in a chain are referenced at least once */
      LWIP_ASSERT("pbuf_free: p->ref > 0", p->ref > 0);
      /* decrease reference count (number of pointers to pbuf) */
      ref = --(p->ref);
      SYS_ARCH_UNPROTECT(old_level);
      /* this pbuf is no longer referenced to? */
      if (ref == 0) {
        /* remember next pbuf in chain for next iteration */
        q = p->next;
        LWIP_DEBUGF( PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free: deallocating %p\r\n", (void *)p));
        type = p->type;
  #if LWIP_SUPPORT_CUSTOM_PBUF
        /* is this a custom pbuf? */
        if ((p->flags & PBUF_FLAG_IS_CUSTOM) != 0) {
          struct pbuf_custom *pc = (struct pbuf_custom*)p;
          LWIP_ASSERT("pc->custom_free_function != NULL", pc->custom_free_function != NULL);
          pc->custom_free_function(p);
        } else
  #endif /* LWIP_SUPPORT_CUSTOM_PBUF */
  
  //Free Buffer
        {
          /* is this a pbuf from the pool? */
          if (type == PBUF_POOL || type == PBUF_RAW) {
            OS_MemFree((void*)p);
          /* is this a ROM or RAM referencing pbuf? */
          } else if (type == PBUF_ROM || type == PBUF_REF) {
            memp_free(MEMP_PBUF, (void*)p);
            
           LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE,("free ROM/REF\r\n"));
          /* type == PBUF_RAM */
          } else {
            mem_free((void*)p);
  
           LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE,("free RAM\r\n"));
  
          }
        }
        count++;
  
  
   //     if(count >= 2)
   //       printf("more than one pbuf in the chain....\r\n");
  
  
        
        /* proceed to next pbuf */
        p = q;
      /* p->ref > 0, this pbuf is still referenced to */
      /* (and so the remaining pbufs in chain as well) */
      } else {
        LWIP_DEBUGF( PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_free: %p has ref %"U16_F", ending here.\r\n", (void *)p, ref));
        /* stop walking through the chain */
        p = NULL;
      }
    }
    PERF_STOP("pbuf_free");
    /* return number of de-allocated pbufs */
    return count;
  }
  
  /**
   * Count number of pbufs in a chain
   *
   * @param p first pbuf of chain
   * @return the number of pbufs in a chain
   */
  
  u8_t
  pbuf_clen(struct pbuf *p)
  {
    u8_t len;
  
    len = 0;
    while (p != NULL) {
      ++len;
      p = p->next;
    }
    return len;
  }
  
  /**
   * Increment the reference count of the pbuf.
   *
   * @param p pbuf to increase reference counter of
   *
   */
  void
  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_ERROR("(h != NULL) && (t != NULL) (programmer violates API)",
               ((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 | LWIP_DBG_TRACE, ("pbuf_chain: %p references %p\r\n", (void *)h, (void *)t));
  }
  
  /**
   * 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 | LWIP_DBG_TRACE, ("pbuf_dechain: unreferencing %p\r\n", (void *)q));
      tail_gone = pbuf_free(q);
      if (tail_gone > 0) {
        LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE,
                    ("pbuf_dechain: deallocated %p (as it is no longer referenced)\r\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);
  }
  
  /**
   *
   * Create PBUF_RAM copies of pbufs.
   *
   * Used to queue packets on behalf of the lwIP stack, such as
   * ARP based queueing.
   *
   * @note You MUST explicitly use p = pbuf_take(p);
   *
   * @note Only one packet is copied, no packet queue!
   *
   * @param p_to pbuf destination of the copy
   * @param p_from pbuf source of the copy
   *
   * @return ERR_OK if pbuf was copied
   *         ERR_ARG if one of the pbufs is NULL or p_to is not big
   *                 enough to hold p_from
   */
  err_t
  pbuf_copy(struct pbuf *p_to, struct pbuf *p_from)
  {
    u16_t offset_to=0, offset_from=0, len;
  
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_copy(%p, %p)\r\n",
      (void*)p_to, (void*)p_from));
  
    /* is the target big enough to hold the source? */
    LWIP_ERROR("pbuf_copy: target not big enough to hold source", ((p_to != NULL) &&
               (p_from != NULL) && (p_to->tot_len >= p_from->tot_len)), return ERR_ARG;);
  
    /* iterate through pbuf chain */
    do
    {
      /* copy one part of the original chain */
      if ((p_to->len - offset_to) >= (p_from->len - offset_from)) {
        /* complete current p_from fits into current p_to */
        len = p_from->len - offset_from;
      } else {
        /* current p_from does not fit into current p_to */
        len = p_to->len - offset_to;
      }
      MEMCPY((u8_t*)p_to->payload + offset_to, (u8_t*)p_from->payload + offset_from, len);
      offset_to += len;
      offset_from += len;
      LWIP_ASSERT("offset_to <= p_to->len", offset_to <= p_to->len);
      LWIP_ASSERT("offset_from <= p_from->len", offset_from <= p_from->len);
      if (offset_from >= p_from->len) {
        /* on to next p_from (if any) */
        offset_from = 0;
        p_from = p_from->next;
      }
      if (offset_to == p_to->len) {
        /* on to next p_to (if any) */
        offset_to = 0;
        p_to = p_to->next;
        LWIP_ERROR("p_to != NULL", (p_to != NULL) || (p_from == NULL) , return ERR_ARG;);
      }
  
      if((p_from != NULL) && (p_from->len == p_from->tot_len)) {
        /* don't copy more than one packet! */
        LWIP_ERROR("pbuf_copy() does not allow packet queues!\r\n",
                   (p_from->next == NULL), return ERR_VAL;);
      }
      if((p_to != NULL) && (p_to->len == p_to->tot_len)) {
        /* don't copy more than one packet! */
        LWIP_ERROR("pbuf_copy() does not allow packet queues!\r\n",
                    (p_to->next == NULL), return ERR_VAL;);
      }
    } while (p_from);
    LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_copy: end of chain reached.\r\n"));
    return ERR_OK;
  }
  
  /**
   * Copy (part of) the contents of a packet buffer
   * to an application supplied buffer.
   *
   * @param buf the pbuf from which to copy data
   * @param dataptr the application supplied buffer
   * @param len length of data to copy (dataptr must be big enough). No more 
   * than buf->tot_len will be copied, irrespective of len
   * @param offset offset into the packet buffer from where to begin copying len bytes
   * @return the number of bytes copied, or 0 on failure
   */
  u16_t
  pbuf_copy_partial(struct pbuf *buf, void *dataptr, u16_t len, u16_t offset)
  {
    struct pbuf *p;
    u16_t left;
    u16_t buf_copy_len;
    u16_t copied_total = 0;
  
    LWIP_ERROR("pbuf_copy_partial: invalid buf", (buf != NULL), return 0;);
    LWIP_ERROR("pbuf_copy_partial: invalid dataptr", (dataptr != NULL), return 0;);
  
    left = 0;
  
    if((buf == NULL) || (dataptr == NULL)) {
      return 0;
    }
  
    /* Note some systems use byte copy if dataptr or one of the pbuf payload pointers are unaligned. */
    for(p = buf; len != 0 && p != NULL; p = p->next) {
      if ((offset != 0) && (offset >= p->len)) {
        /* don't copy from this buffer -> on to the next */
        offset -= p->len;
      } else {
        /* copy from this buffer. maybe only partially. */
        buf_copy_len = p->len - offset;
        if (buf_copy_len > len)
            buf_copy_len = len;
        /* copy the necessary parts of the buffer */
        MEMCPY(&((char*)dataptr)[left], &((char*)p->payload)[offset], buf_copy_len);
        copied_total += buf_copy_len;
        left += buf_copy_len;
        len -= buf_copy_len;
        offset = 0;
      }
    }
    return copied_total;
  }
  
  /**
   * Copy application supplied data into a pbuf.
   * This function can only be used to copy the equivalent of buf->tot_len data.
   *
   * @param buf pbuf to fill with data
   * @param dataptr application supplied data buffer
   * @param len length of the application supplied data buffer
   *
   * @return ERR_OK if successful, ERR_MEM if the pbuf is not big enough
   */
  err_t
  pbuf_take(struct pbuf *buf, const void *dataptr, u16_t len)
  {
    struct pbuf *p;
    u16_t buf_copy_len;
    u16_t total_copy_len = len;
    u16_t copied_total = 0;
  
    LWIP_ERROR("pbuf_take: invalid buf", (buf != NULL), return 0;);
    LWIP_ERROR("pbuf_take: invalid dataptr", (dataptr != NULL), return 0;);
  
    if ((buf == NULL) || (dataptr == NULL) || (buf->tot_len < len)) {
      return ERR_ARG;
    }
  
    /* Note some systems use byte copy if dataptr or one of the pbuf payload pointers are unaligned. */
    for(p = buf; total_copy_len != 0; p = p->next) {
      LWIP_ASSERT("pbuf_take: invalid pbuf", p != NULL);
      buf_copy_len = total_copy_len;
      if (buf_copy_len > p->len) {
        /* this pbuf cannot hold all remaining data */
        buf_copy_len = p->len;
      }
      /* copy the necessary parts of the buffer */
      MEMCPY(p->payload, &((char*)dataptr)[copied_total], buf_copy_len);
      total_copy_len -= buf_copy_len;
      copied_total += buf_copy_len;
    }
    LWIP_ASSERT("did not copy all data", total_copy_len == 0 && copied_total == len);
    return ERR_OK;
  }
  
  /**
   * Creates a single pbuf out of a queue of pbufs.
   *
   * @remark: Either the source pbuf 'p' is freed by this function or the original
   *          pbuf 'p' is returned, therefore the caller has to check the result!
   *
   * @param p the source pbuf
   * @param layer pbuf_layer of the new pbuf
   *
   * @return a new, single pbuf (p->next is NULL)
   *         or the old pbuf if allocation fails
   */
  struct pbuf*
  pbuf_coalesce(struct pbuf *p, pbuf_layer layer)
  {
    struct pbuf *q;
    err_t err;
    if (p->next == NULL) {
      return p;
    }
    q = pbuf_alloc(layer, p->tot_len, PBUF_RAM);
    if (q == NULL) {
      /* @todo: what do we do now? */
      return p;
    }
    err = pbuf_copy(q, p);
    LWIP_ASSERT("pbuf_copy failed", err == ERR_OK);
    LWIP_UNUSED_ARG(err);
    pbuf_free(p);
    return q;
  }
  
  #if LWIP_CHECKSUM_ON_COPY
  /**
   * Copies data into a single pbuf (*not* into a pbuf queue!) and updates
   * the checksum while copying
   *
   * @param p the pbuf to copy data into
   * @param start_offset offset of p->payload where to copy the data to
   * @param dataptr data to copy into the pbuf
   * @param len length of data to copy into the pbuf
   * @param chksum pointer to the checksum which is updated
   * @return ERR_OK if successful, another error if the data does not fit
   *         within the (first) pbuf (no pbuf queues!)
   */
  err_t
  pbuf_fill_chksum(struct pbuf *p, u16_t start_offset, const void *dataptr,
                   u16_t len, u16_t *chksum)
  {
    u32_t acc;
    u16_t copy_chksum;
    char *dst_ptr;
    LWIP_ASSERT("p != NULL", p != NULL);
    LWIP_ASSERT("dataptr != NULL", dataptr != NULL);
    LWIP_ASSERT("chksum != NULL", chksum != NULL);
    LWIP_ASSERT("len != 0", len != 0);
  
    if ((start_offset >= p->len) || (start_offset + len > p->len)) {
      return ERR_ARG;
    }
  
    dst_ptr = ((char*)p->payload) + start_offset;
    copy_chksum = LWIP_CHKSUM_COPY(dst_ptr, dataptr, len);
    if ((start_offset & 1) != 0) {
      copy_chksum = SWAP_BYTES_IN_WORD(copy_chksum);
    }
    acc = *chksum;
    acc += copy_chksum;
    *chksum = FOLD_U32T(acc);
    return ERR_OK;
  }
  #endif /* LWIP_CHECKSUM_ON_COPY */
  
   /** Get one byte from the specified position in a pbuf
   * WARNING: returns zero for offset >= p->tot_len
   *
   * @param p pbuf to parse
   * @param offset offset into p of the byte to return
   * @return byte at an offset into p OR ZERO IF 'offset' >= p->tot_len
   */
  u8_t
  pbuf_get_at(struct pbuf* p, u16_t offset)
  {
    u16_t copy_from = offset;
    struct pbuf* q = p;
  
    /* get the correct pbuf */
    while ((q != NULL) && (q->len <= copy_from)) {
      copy_from -= q->len;
      q = q->next;
    }
    /* return requested data if pbuf is OK */
    if ((q != NULL) && (q->len > copy_from)) {
      return ((u8_t*)q->payload)[copy_from];
    }
    return 0;
  }
  
  /** Compare pbuf contents at specified offset with memory s2, both of length n
   *
   * @param p pbuf to compare
   * @param offset offset into p at wich to start comparing
   * @param s2 buffer to compare
   * @param n length of buffer to compare
   * @return zero if equal, nonzero otherwise
   *         (0xffff if p is too short, diffoffset+1 otherwise)
   */
  u16_t
  pbuf_memcmp(struct pbuf* p, u16_t offset, const void* s2, u16_t n)
  {
    u16_t start = offset;
    struct pbuf* q = p;
  
    /* get the correct pbuf */
    while ((q != NULL) && (q->len <= start)) {
      start -= q->len;
      q = q->next;
    }
    /* return requested data if pbuf is OK */
    if ((q != NULL) && (q->len > start)) {
      u16_t i;
      for(i = 0; i < n; i++) {
        u8_t a = pbuf_get_at(q, start + i);
        u8_t b = ((u8_t*)s2)[i];
        if (a != b) {
          return i+1;
        }
      }
      return 0;
    }
    return 0xffff;
  }
  
  /** Find occurrence of mem (with length mem_len) in pbuf p, starting at offset
   * start_offset.
   *
   * @param p pbuf to search, maximum length is 0xFFFE since 0xFFFF is used as
   *        return value 'not found'
   * @param mem search for the contents of this buffer
   * @param mem_len length of 'mem'
   * @param start_offset offset into p at which to start searching
   * @return 0xFFFF if substr was not found in p or the index where it was found
   */
  u16_t
  pbuf_memfind(struct pbuf* p, const void* mem, u16_t mem_len, u16_t start_offset)
  {
    u16_t i;
    u16_t max = p->tot_len - mem_len;
    if (p->tot_len >= mem_len + start_offset) {
      for(i = start_offset; i <= max; ) {
        u16_t plus = pbuf_memcmp(p, i, mem, mem_len);
        if (plus == 0) {
          return i;
        } else {
          i += plus;
        }
      }
    }
    return 0xFFFF;
  }
  
  /** Find occurrence of substr with length substr_len in pbuf p, start at offset
   * start_offset
   * WARNING: in contrast to strstr(), this one does not stop at the first \0 in
   * the pbuf/source string!
   *
   * @param p pbuf to search, maximum length is 0xFFFE since 0xFFFF is used as
   *        return value 'not found'
   * @param substr string to search for in p, maximum length is 0xFFFE
   * @return 0xFFFF if substr was not found in p or the index where it was found
   */
  u16_t
  pbuf_strstr(struct pbuf* p, const char* substr)
  {
    size_t substr_len;
    if ((substr == NULL) || (substr[0] == 0) || (p->tot_len == 0xFFFF)) {
      return 0xFFFF;
    }
    substr_len = strlen(substr);
    if (substr_len >= 0xFFFF) {
      return 0xFFFF;
    }
    return pbuf_memfind(p, substr, (u16_t)substr_len, 0);
  }
  