lwip PBUF_POOL类型pbuf申请

maqiufeng

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

  switch (type) {
    case PBUF_REF: /* fall through */
    case PBUF_ROM:
      p = pbuf_alloc_reference(NULL, length, type);
      break;
    case PBUF_POOL: {
      struct pbuf *q, *last;
      u16_t rem_len; /* remaining length */
      p = NULL;
      last = NULL;
      rem_len = length;
      do {
        u16_t qlen;
        q = (struct pbuf *)memp_malloc(MEMP_PBUF_POOL);
        if (q == NULL) {
          PBUF_POOL_IS_EMPTY();
          /* free chain so far allocated */
          if (p) {
            pbuf_free(p);
          }
          /* bail out unsuccessfully */
          return NULL;
        }
        qlen = LWIP_MIN(rem_len, (u16_t)(PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)));
        pbuf_init_alloced_pbuf(q, LWIP_MEM_ALIGN((void *)((u8_t *)q + SIZEOF_STRUCT_PBUF + offset)),
                               rem_len, qlen, type, 0);
        LWIP_ASSERT("pbuf_alloc: pbuf q->payload properly aligned",
                    ((mem_ptr_t)q->payload % MEM_ALIGNMENT) == 0);
        LWIP_ASSERT("PBUF_POOL_BUFSIZE must be bigger than MEM_ALIGNMENT",
                    (PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset)) > 0 );
        if (p == NULL) {
          /* allocated head of pbuf chain (into p) */
          p = q;
        } else {
          /* make previous pbuf point to this pbuf */
          last->next = q;
        }
        last = q;
        rem_len = (u16_t)(rem_len - qlen);
        offset = 0;
      } while (rem_len > 0);
      break;
    }
    case PBUF_RAM: {
      mem_size_t payload_len = (mem_size_t)(LWIP_MEM_ALIGN_SIZE(offset) + LWIP_MEM_ALIGN_SIZE(length));
      mem_size_t alloc_len = (mem_size_t)(LWIP_MEM_ALIGN_SIZE(SIZEOF_STRUCT_PBUF) + payload_len);

      /* bug #50040: Check for integer overflow when calculating alloc_len */
      if ((payload_len < LWIP_MEM_ALIGN_SIZE(length)) ||
          (alloc_len < LWIP_MEM_ALIGN_SIZE(length))) {
        return NULL;
      }

      /* If pbuf is to be allocated in RAM, allocate memory for it. */
      p = (struct pbuf *)mem_malloc(alloc_len);
      if (p == NULL) {
        return NULL;
      }
      pbuf_init_alloced_pbuf(p, LWIP_MEM_ALIGN((void *)((u8_t *)p + SIZEOF_STRUCT_PBUF + offset)),
                             length, length, type, 0);
      LWIP_ASSERT("pbuf_alloc: pbuf->payload properly aligned",
                  ((mem_ptr_t)p->payload % MEM_ALIGNMENT) == 0);
      break;
    }
    default:
      LWIP_ASSERT("pbuf_alloc: erroneous type", 0);
      return NULL;
  }
  LWIP_DEBUGF(PBUF_DEBUG | LWIP_DBG_TRACE, ("pbuf_alloc(length=%"U16_F") == %p\n", length, (void *)p));
  return p;
}

qlen=LWIP_MIN(rem_len, (u16_t)(PBUF_POOL_BUFSIZE_ALIGNED - LWIP_MEM_ALIGN_SIZE(offset))); 例如rem_len 1500字节 PBUF_POOL_BUFSIZE_ALIGNED 512字节 LWIP_MEM_ALIGN_SIZE(offset) 54字节
则 qlen=LWIP_MIN（1500，512-54）=458字节。pbuf_init_alloced_pbuf(q, LWIP_MEM_ALIGN((void *)((u8_t *)q + SIZEOF_STRUCT_PBUF + offset)), rem_len, qlen, type, 0);其中LWIP_MEM_ALIGN((void *)((u8_t *)q + SIZEOF_STRUCT_PBUF + offset) 也就是payload指向的数据区域是经过加上SIZEOF_STRUCT_PBUF 偏移的 那为什么计算数据区域大小qlen=LWIP_MIN（1500，512-54）=458字节时不需要减去结构体的长度呢512-54-SIZEOF_STRUCT_PBUF？

maqiufeng

问题已找到
LWIP_PBUF_MEMPOOL(PBUF_POOL, PBUF_POOL_SIZE,           PBUF_POOL_BUFSIZE,             "PBUF_POOL")
LWIP_PBUF_MEMPOOL(name, num, payload, desc) LWIP_MEMPOOL(name, num, (LWIP_MEM_ALIGN_SIZE(sizeof(struct pbuf)) + LWIP_MEM_ALIGN_SIZE(payload)), desc) 已经分配了结构体的空间