LWIP学习

转自：https://blog.csdn.net/kzq_qmi/article/details/46900589

数据包pbuf： 
　　 
　　LwIP采用数据结构 pbuf 来描述数据包，其结构如下： 
　　 
　　

struct pbuf {
  /** next pbuf in singly linked pbuf chain */
  struct pbuf *next;

  /** pointer to the actual data in the buffer */
  void *payload;

  /**
   * total length of this buffer and all next buffers in chain
   * belonging to the same packet.
   *
   * For non-queue packet chains this is the invariant:
   * p->tot_len == p->len + (p->next? p->next->tot_len: 0)
   */
  u16_t tot_len;

  /** length of this buffer */
  u16_t len;

  /** pbuf_type as u8_t instead of enum to save space */
  u8_t /*pbuf_type*/ type;

  /** misc flags */
  u8_t flags;

  /**
   * the reference count always equals the number of pointers
   * that refer to this pbuf. This can be pointers from an application,
   * the stack itself, or pbuf->next pointers from a chain.
   */
  u16_t ref;
};

　　各成员含义上面的注释已经说得很清楚了。 
　　关于采用链表结构，是因为实际发送或接收的数据包可能很大，而每个 pbuf 能够管理的数据可能很少，所以，往往需要多个 pbuf 结构才能完全描述一个数据包。 
　　另外，最后的 ref 字段表示该 pbuf 被引用的次数。这里又是一个纠结的地方啊。初始化一个 pbuf 的时候， ref 字段值被设置为 1，当有其他 pbuf 的 next 指针指向该 pbuf 时，该 pbuf 的 ref 字段值加一。所以，要删除一个 pbuf 时， ref 的值必须为 1 才能删除成功，否则删除失败。 
　　上图中注意 payload 并没有指向 ref 字段之后，而是隔了一定的区域。这段区域就是offset 的大小，这段区域用来存储数据的包头，如 TCP 包头， IP 包头等。当然， offset 也可以是 0。

来看代码：

/**
 * 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.
 */

struct pbuf *
pbuf_alloc(pbuf_layer layer, u16_t length, pbuf_type type)
{
  struct pbuf *p, *q, *r;
  u16_t offset;
  s32_t rem_len; /* remaining length */
  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F")\n", length));

  /* determine header offset */
  offset = 0;
  switch (layer) { //注意这里从协议栈上层开始，方便offset从上层往下叠加，因此也没加 break
  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 (type) {
  case PBUF_POOL:
    /* allocate head of pbuf chain into p */
    p = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL); //分配第一个pbuf

    if (p == NULL) {
      return NULL;
    }
    p->type = type;
    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)));

    /* the total length of the pbuf chain is the requested size */
    p->tot_len = length; //该pbuf及其以后pbuf的负载数据总长度
    /* set the length of the first pbuf in the chain */
    p->len = LWIP_MIN(length, PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)); //负载数据可能大于分配空间长度，也有可能小于，取当前pbuf实际的负载长度

    /* set reference count (needed here in case we fail) */
    p->ref = 1;

    /* now allocate the tail of the pbuf chain */
    //如果一个pbuf不够的话，接着分配
    /* 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 = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL); //从第二个pbuf开始，不再需要TCP/IP之类的头，所以没有offset
      if (q == NULL) {
        /* free chain so far allocated */
        pbuf_free(p);     //注意这里，如果当前pbuf分配不成功，要把之前分配的所有pbuf都释放掉
        /* bail out unsuccesfully */
        return NULL;
      }
      q->type = 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 */
      q->tot_len = (u16_t)rem_len;
      /* this pbuf length is pool size, unless smaller sized tail */
      q->len = LWIP_MIN((u16_t)rem_len, PBUF_POOL_BUFSIZE_ALIGNED);
      q->payload = (void *)((u8_t *)q + SIZEOF_STRUCT_PBUF);

      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 = (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;
    break;
  /* 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 */
    p = (struct pbuf *)memp_malloc(MEMP_PBUF);
    if (p == NULL) {
      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:
    return NULL;
  }
  /* set reference count */
  p->ref = 1;
  /* set flags */
  p->flags = 0;
  return p;
}

/**
 * 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) {
    return 0;
  }

  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); //申请临界变量保护
    /* 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);   //进入临界区
    /* 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); //退出临界区

    /* this pbuf is no longer referenced to? */
    if (ref == 0) {
      /* remember next pbuf in chain for next iteration */
      q = p->next;

      type = p->type;

      /* is this a pbuf from the pool? */
      if (type == PBUF_POOL) {
        memp_free(MEMP_PBUF_POOL, p);
      /* is this a ROM or RAM referencing pbuf? */
      } else if (type == PBUF_ROM || type == PBUF_REF) {
        memp_free(MEMP_PBUF, p);
      /* type == PBUF_RAM */
      } else {
        mem_free(p);
      }

      count++;
      /* 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 {
      /* stop walking through the chain */
      p = NULL;
    }
  }
  /* return number of de-allocated pbufs */
  return count;
}

/**
 *
 * 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;

  /* 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
  {
    LWIP_ASSERT("p_to != NULL", p_to != NULL);

    /* copy one part of the original chain */
    if ((p_to->len - offset_to) >= (p_from->len - offset_from)) { //每次拷贝的长度是源端和目标端当前pbuf所剩空间的较小值，offset为当前pbuf拷贝数据的偏移量
      /* 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;

    if (offset_to == p_to->len) { //目标端当前pbuf空间已满，转向下一个pbuf,记得offset清零
      /* on to next p_to (if any) */
      offset_to = 0;
      p_to = p_to->next;
    }

    if (offset_from >= p_from->len) { //源端当前pbuf数据已拷贝完，转向下一个pbuf,记得offset清零
      /* on to next p_from (if any) */
      offset_from = 0;
      p_from = p_from->next;
    }

    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!\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!\n",
                  (p_to->next == NULL), return ERR_VAL;);
    }
  } while (p_from);

  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_copy: end of chain reached.\n"));
  return ERR_OK;
}

　　可以看到，回收 pbuf 使用pbuf_free()函数，该函数首先要减少 pbuf 索引计数（reference count）。如果引用计数已经减为 0，这个 pbuf 被回收。对于一个pbuf链来说，只有前一个pbuf被回收，才会考虑回收后面的pbuf，如果前面pbuf计数还不为0，则直接返回。