LwIP 协议栈源码分析(读书笔记)--内存管理--动态内存堆(HEAP)分配策略源码

本章分析动态内存堆(HEAP)的第一种分配策略，承接上篇文章
LwIP 协议栈源码分析(读书笔记)–内存管理–机制策略分析

组织形式：

mem.c源码解析如下：

mem_init():

/**
 * Zero the heap and initialize start, end and lowest-free
 */
void
mem_init(void)
{
  struct mem *mem;

  LWIP_ASSERT("Sanity check alignment",
    (SIZEOF_STRUCT_MEM & (MEM_ALIGNMENT-1)) == 0);//内存大小对齐及检测

  /* align the heap */
  ram = (u8_t *)LWIP_MEM_ALIGN(LWIP_RAM_HEAP_POINTER);//堆地址 对齐
  /* initialize the start of the heap==初始化堆链表的头结点 */
  mem = (struct mem *)(void *)ram;//指针类型强转为链表
  mem->next = MEM_SIZE_ALIGNED;//#define MEM_SIZE  16000 //内存堆heap大小
  mem->prev = 0;
  mem->used = 0;
  /* initialize the end of the heap==初始化堆链表的结束结点 */
  ram_end = (struct mem *)(void *)&ram[MEM_SIZE_ALIGNED];
  ram_end->used = 1;//指明用户不可用
  ram_end->next = MEM_SIZE_ALIGNED;
  ram_end->prev = MEM_SIZE_ALIGNED;

  /* initialize the lowest-free pointer to the start of the heap==
  **指向最低空闲块的指针，这用于更快的搜索 */
  lfree = (struct mem *)(void *)ram;

  MEM_STATS_AVAIL(avail, MEM_SIZE_ALIGNED);

  if(sys_mutex_new(&mem_mutex) != ERR_OK) {//new一个互斥信号量 mem_mutex，名字看出，内存管理专用
    LWIP_ASSERT("failed to create mem_mutex", 0);
  }
}
}

内存对齐操作：SIZEOF_STRUCT_MEM

#define SIZEOF_STRUCT_MEM    LWIP_MEM_ALIGN_SIZE(sizeof(struct mem))

关于字节对齐算法:LWIP_MEM_ALIGN_SIZE()
参见：字节对齐算法
进行此项操作的理由：lwip按自己的内存对齐规则管理维护内部的数据，不受编译器的对齐规则影响。

/** pointer to the heap (ram_heap): for alignment, ram is now a pointer instead of an array== 堆起始地址*/
static u8_t *ram;
/** the last entry, always unused! ==最后一项，不会被使用*/
static struct mem *ram_end;
/** pointer to the lowest free block, this is used for faster search *==指向最低空闲块的指针，这用于更快的搜索*/
static struct mem *lfree;

mem_malloc( ) ：

/**
 * Adam's mem_malloc() plus solution for bug #17922
 * Allocate a block of memory with a minimum of 'size' bytes.
 *
 * @param size is the minimum size of the requested block in bytes.
 * @return pointer to allocated memory or NULL if no free memory was found.
 *
 * Note that the returned value will always be aligned (as defined by MEM_ALIGNMENT).
 */
void *
mem_malloc(mem_size_t size)
{
  mem_size_t ptr, ptr2;
  struct mem *mem, *mem2;
#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
  u8_t local_mem_free_count = 0;
#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  LWIP_MEM_ALLOC_DECL_PROTECT();//

  if (size == 0) {
    return NULL;
  }

  /* Expand the size of the allocated memory region so that we can
     adjust for alignment. */
  size = LWIP_MEM_ALIGN_SIZE(size);//对齐

  if(size < MIN_SIZE_ALIGNED) {
    /* every data block must be at least MIN_SIZE_ALIGNED long */
    size = MIN_SIZE_ALIGNED;
  }

  if (size > MEM_SIZE_ALIGNED) {
    return NULL;
  }

  /* protect the heap from concurrent access */
  sys_mutex_lock(&mem_mutex);//保护堆不受并发访问
  LWIP_MEM_ALLOC_PROTECT();
#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT//未定义,无do--while
  /* run as long as a mem_free disturbed mem_malloc or mem_trim */
  do {
    local_mem_free_count = 0;
#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */

    /* Scan through the heap searching for a free block that is big enough,
     * beginning with the lowest free block.
     */
    for (ptr = (mem_size_t)((u8_t *)lfree - ram)/*上次分配内存后的最低lfree节点地址*/; ptr < MEM_SIZE_ALIGNED - size/*分配后剩余大小*/;
         ptr = ((struct mem *)(void *)&ram[ptr])->next/*指向下一个结点*/) {
      mem = (struct mem *)(void *)&ram[ptr];

      if ((!mem->used) &&//该块内存未使用
          (mem->next - (ptr + SIZEOF_STRUCT_MEM)) >= size/*相应的内存大小够用*/) {
        /* mem is not used and at least perfect fit is possible:
         * mem->next - (ptr + SIZEOF_STRUCT_MEM) gives us the 'user data size' of mem */

        if (mem->next - (ptr + SIZEOF_STRUCT_MEM) >= (size + SIZEOF_STRUCT_MEM + MIN_SIZE_ALIGNED)) {//是否能够拆分该节点的数据块
          /* (in addition to the above, we test if another struct mem (SIZEOF_STRUCT_MEM) containing
           * at least MIN_SIZE_ALIGNED of data also fits in the 'user data space' of 'mem')
           * -> split large block, create empty remainder,
           * remainder must be large enough to contain MIN_SIZE_ALIGNED data: if
           * mem->next - (ptr + (2*SIZEOF_STRUCT_MEM)) == size,
           * struct mem would fit in but no data between mem2 and mem2->next
           * @todo we could leave out MIN_SIZE_ALIGNED. We would create an empty
           *       region that couldn't hold data, but when mem->next gets freed,
           *       the 2 regions would be combined, resulting in more free memory
           */
          ptr2 = ptr + SIZEOF_STRUCT_MEM + size;//分割内存后，剩余部分的地址
          /* create mem2 struct//新建一个结点维护剩余的内存 */
          mem2 = (struct mem *)(void *)&ram[ptr2];
          mem2->used = 0;
          mem2->next = mem->next;
          mem2->prev = ptr;
          /* and insert it between mem and mem->next */
          mem->next = ptr2;
          mem->used = 1;

          if (mem2->next != MEM_SIZE_ALIGNED) {//非最后一个结点(中间结点)
            ((struct mem *)(void *)&ram[mem2->next])->prev = ptr2;//更新后边的prev地址,完成链表的插入
          }
          MEM_STATS_INC_USED(used, (size + SIZEOF_STRUCT_MEM));
        } else {//不能够拆分，更新used成员为已使用
          /* (a mem2 struct does no fit into the user data space of mem and mem->next will always
           * be used at this point: if not we have 2 unused structs in a row, plug_holes should have
           * take care of this).
           * -> near fit or excact fit: do not split, no mem2 creation
           * also can't move mem->next directly behind mem, since mem->next
           * will always be used at this point!
           */
          mem->used = 1;
          MEM_STATS_INC_USED(used, mem->next - (mem_size_t)((u8_t *)mem - ram));
        }
        if (mem == lfree) {//查找空闲的 没有被使用的节点,方便下次调用
          struct mem *cur = lfree;
          /* Find next free block after mem and update lowest free pointer */
          while (cur->used && cur != ram_end) {
#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
            mem_free_count = 0;
            LWIP_MEM_ALLOC_UNPROTECT();
            /* prevent high interrupt latency... */
            LWIP_MEM_ALLOC_PROTECT();
            if (mem_free_count != 0) {
              /* If mem_free or mem_trim have run, we have to restart since they
                 could have altered our current struct mem or lfree. */
              goto mem_malloc_adjust_lfree;
            }
#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
            cur = (struct mem *)(void *)&ram[cur->next];
          }
          lfree = cur;//更新lfree
          LWIP_ASSERT("mem_malloc: !lfree->used", ((lfree == ram_end) || (!lfree->used)));
        }
        LWIP_MEM_ALLOC_UNPROTECT();
        sys_mutex_unlock(&mem_mutex);//解锁内存互斥信号量
        LWIP_ASSERT("mem_malloc: allocated memory not above ram_end.",
         (mem_ptr_t)mem + SIZEOF_STRUCT_MEM + size <= (mem_ptr_t)ram_end);//边界检查
        LWIP_ASSERT("mem_malloc: allocated memory properly aligned.",
         ((mem_ptr_t)mem + SIZEOF_STRUCT_MEM) % MEM_ALIGNMENT == 0);//对齐检查
        LWIP_ASSERT("mem_malloc: sanity check alignment",
          (((mem_ptr_t)mem) & (MEM_ALIGNMENT-1)) == 0);//对齐检查

        return (u8_t *)mem + SIZEOF_STRUCT_MEM;
      }
    }
#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
    /* if we got interrupted by a mem_free, try again */
  } while(local_mem_free_count != 0);
#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("mem_malloc: could not allocate %"S16_F" bytes\n", (s16_t)size));
  MEM_STATS_INC(err);
  LWIP_MEM_ALLOC_UNPROTECT();
  sys_mutex_unlock(&mem_mutex);
  return NULL;
}

流程描述：
从lfree空闲块节点开始，确认used成员，比较节点内存时候够用，否则向后查找。
一旦找到，判断下能够分割，
能分割，意味着需要插入一个节点到链表。
否则，更新used成员为已使用。
找到合适的内存后，更新lfree空闲块儿指针；
检查对齐，
return 数据内存地址(需要去掉维护链表节点的内存)

mem_calloc( )：

/**
 * Contiguously allocates enough space for count objects that are size bytes
 * of memory each and returns a pointer to the allocated memory.
 *
 * The allocated memory is filled with bytes of value zero.
 *
 * @param count number of objects to allocate
 * @param size size of the objects to allocate
 * @return pointer to allocated memory / NULL pointer if there is an error
 */
void *mem_calloc(mem_size_t count, mem_size_t size)
{
  void *p;

  /* allocate 'count' objects of size 'size' */
  p = mem_malloc(count * size);//分配内存
  if (p) {
    /* zero the memory */
    memset(p, 0, count * size);//清零
  }
  return p;
}

对mem_malloc进行封装====清零已分配的内存。

mem_free（）：

/**
 * Put a struct mem back on the heap
 *
 * @param rmem is the data portion of a struct mem as returned by a previous
 *             call to mem_malloc()
 */
void
mem_free(void *rmem)
{
  struct mem *mem;
  LWIP_MEM_FREE_DECL_PROTECT();

  if (rmem == NULL) {
    LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_TRACE | LWIP_DBG_LEVEL_SERIOUS, ("mem_free(p == NULL) was called.\n"));
    return;
  }
  LWIP_ASSERT("mem_free: sanity check alignment", (((mem_ptr_t)rmem) & (MEM_ALIGNMENT-1)) == 0);

  LWIP_ASSERT("mem_free: legal memory", (u8_t *)rmem >= (u8_t *)ram &&
    (u8_t *)rmem < (u8_t *)ram_end);

  if ((u8_t *)rmem < (u8_t *)ram || (u8_t *)rmem >= (u8_t *)ram_end) {
    SYS_ARCH_DECL_PROTECT(lev);
    LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SEVERE, ("mem_free: illegal memory\n"));
    /* protect mem stats from concurrent access */
    SYS_ARCH_PROTECT(lev);
    MEM_STATS_INC(illegal);
    SYS_ARCH_UNPROTECT(lev);
    return;
  }
  /* protect the heap from concurrent access */
  LWIP_MEM_FREE_PROTECT();
  /* Get the corresponding struct mem ... */
  mem = (struct mem *)(void *)((u8_t *)rmem - SIZEOF_STRUCT_MEM);//确定相关链表节点
  /* ... which has to be in a used state ... */
  LWIP_ASSERT("mem_free: mem->used", mem->used);
  /* ... and is now unused. */
  mem->used = 0;//更新该块内存为未使用

  if (mem < lfree) {//更新lfree
    /* the newly freed struct is now the lowest */
    lfree = mem;
  }

  MEM_STATS_DEC_USED(used, mem->next - (mem_size_t)(((u8_t *)mem - ram)));

  /* finally, see if prev or next are free also */
  plug_holes(mem);//整合相邻的节点,满足条件则合并
#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
  mem_free_count = 1;
#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  LWIP_MEM_FREE_UNPROTECT();
}

释放内存，并整合相邻的资源。

mem_trim():

/**
 * Shrink memory returned by mem_malloc().
 *
 * @param rmem pointer to memory allocated by mem_malloc the is to be shrinked
 * @param newsize required size after shrinking (needs to be smaller than or
 *                equal to the previous size)
 * @return for compatibility reasons: is always == rmem, at the moment
 *         or NULL if newsize is > old size, in which case rmem is NOT touched
 *         or freed!
 */
void *
mem_trim(void *rmem, mem_size_t newsize)
{
  mem_size_t size;
  mem_size_t ptr, ptr2;
  struct mem *mem, *mem2;
  /* use the FREE_PROTECT here: it protects with sem OR SYS_ARCH_PROTECT */
  LWIP_MEM_FREE_DECL_PROTECT();

  /* Expand the size of the allocated memory region so that we can
     adjust for alignment. */
  newsize = LWIP_MEM_ALIGN_SIZE(newsize);

  if(newsize < MIN_SIZE_ALIGNED) {
    /* every data block must be at least MIN_SIZE_ALIGNED long */
    newsize = MIN_SIZE_ALIGNED;
  }

  if (newsize > MEM_SIZE_ALIGNED) {
    return NULL;
  }

  LWIP_ASSERT("mem_trim: legal memory", (u8_t *)rmem >= (u8_t *)ram &&
   (u8_t *)rmem < (u8_t *)ram_end);

  if ((u8_t *)rmem < (u8_t *)ram || (u8_t *)rmem >= (u8_t *)ram_end) {
    SYS_ARCH_DECL_PROTECT(lev);
    LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SEVERE, ("mem_trim: illegal memory\n"));
    /* protect mem stats from concurrent access */
    SYS_ARCH_PROTECT(lev);
    MEM_STATS_INC(illegal);
    SYS_ARCH_UNPROTECT(lev);
    return rmem;
  }
  /* Get the corresponding struct mem ... */
  mem = (struct mem *)(void *)((u8_t *)rmem - SIZEOF_STRUCT_MEM);//获取输入的rmem内存地址的链表节点
  /* ... and its offset pointer */
  ptr = (mem_size_t)((u8_t *)mem - ram);//索引

  size = mem->next - ptr - SIZEOF_STRUCT_MEM;//节点内存块的大小
  LWIP_ASSERT("mem_trim can only shrink memory", newsize <= size);
  if (newsize > size) {
    /* not supported */
    return NULL;
  }
  if (newsize == size) {
    /* No change in size, simply return */
    return rmem;
  }

  /* protect the heap from concurrent access */
  LWIP_MEM_FREE_PROTECT();

  mem2 = (struct mem *)(void *)&ram[mem->next];//下一个节点
  if(mem2->used == 0) {//空闲,则剩余部分与下一个节点合并
    /* The next struct is unused, we can simply move it at little */
    mem_size_t next;
    /* remember the old next pointer */
    next = mem2->next;
    /* create new struct mem which is moved directly after the shrinked mem */
    ptr2 = ptr + SIZEOF_STRUCT_MEM + newsize;
    if (lfree == mem2) {
      lfree = (struct mem *)(void *)&ram[ptr2];
    }
    mem2 = (struct mem *)(void *)&ram[ptr2];
    mem2->used = 0;
    /* restore the next pointer */
    mem2->next = next;
    /* link it back to mem */
    mem2->prev = ptr;
    /* link mem to it */
    mem->next = ptr2;
    /* last thing to restore linked list: as we have moved mem2,
     * let 'mem2->next->prev' point to mem2 again. but only if mem2->next is not
     * the end of the heap */
    if (mem2->next != MEM_SIZE_ALIGNED) {
      ((struct mem *)(void *)&ram[mem2->next])->prev = ptr2;
    }
    MEM_STATS_DEC_USED(used, (size - newsize));
    /* no need to plug holes, we've already done that */
  } else if (newsize + SIZEOF_STRUCT_MEM + MIN_SIZE_ALIGNED <= size) {//不能合并到下一个节点，新建节点并插入链表
    /* Next struct is used but there's room for another struct mem with
     * at least MIN_SIZE_ALIGNED of data.
     * Old size ('size') must be big enough to contain at least 'newsize' plus a struct mem
     * ('SIZEOF_STRUCT_MEM') with some data ('MIN_SIZE_ALIGNED').
     * @todo we could leave out MIN_SIZE_ALIGNED. We would create an empty
     *       region that couldn't hold data, but when mem->next gets freed,
     *       the 2 regions would be combined, resulting in more free memory */
    ptr2 = ptr + SIZEOF_STRUCT_MEM + newsize;
    mem2 = (struct mem *)(void *)&ram[ptr2];
    if (mem2 < lfree) {
      lfree = mem2;
    }
    mem2->used = 0;
    mem2->next = mem->next;
    mem2->prev = ptr;
    mem->next = ptr2;
    if (mem2->next != MEM_SIZE_ALIGNED) {
      ((struct mem *)(void *)&ram[mem2->next])->prev = ptr2;
    }
    MEM_STATS_DEC_USED(used, (size - newsize));
    /* the original mem->next is used, so no need to plug holes! */
  }
  /* else {
    next struct mem is used but size between mem and mem2 is not big enough
    to create another struct mem
    -> don't do anyhting. 
    -> the remaining space stays unused since it is too small
  } */
#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT
  mem_free_count = 1;
#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */
  LWIP_MEM_FREE_UNPROTECT();
  return rmem;
}

功能：缩小已申请内存的大小，参数rmem务必小于参数newsize
应用场景：减小调用mem_malloc()函数申请的内存大小，
从功能上看，调用mem_malloc(500)申请了500字节，不想用那么多了，还不想mem_free()释放，在当前申请到的内存基础上，重新申请一块更小的。相当于将当前的内存再次分割或者合并到相邻的内存。