Function pbuf_realloc

components/net/lwip/lwip-2.1.2/src/core/pbuf.c:401–459

* @ingroup 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

 * @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 */
  u16_t shrink;

  LWIP_ASSERT("pbuf_realloc: p != NULL", p != NULL);

  /* 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) */
  shrink = (u16_t)(p->tot_len - new_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 = (u16_t)(rem_len - q->len);
    /* decrease total length indicator */
    q->tot_len = (u16_t)(q->tot_len - shrink);
    /* 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 (pbuf_match_allocsrc(q, PBUF_TYPE_ALLOC_SRC_MASK_STD_HEAP) && (rem_len != q->len)
#if LWIP_SUPPORT_CUSTOM_PBUF
      && ((q->flags & PBUF_FLAG_IS_CUSTOM) == 0)
#endif /* LWIP_SUPPORT_CUSTOM_PBUF */
     ) {
    /* reallocate and adjust the length of the pbuf that will be split */
    q = (struct pbuf *)mem_trim(q, (mem_size_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;