1 内存管理模型
此处讨论的是nuttx的堆内存管理，其中内存管理模型如下图所示
2 通过realloc缩小内存
参考函数：nuttx/mm/mm_heap/mm_realloc.c，其中缩小内存直接进入以下分支返回，可以看到返回的仍然是oldmem，内存地址不会改变
FAR void *mm_realloc(FAR struct mm_heap_s *heap, FAR void *oldmem,
size_t size)
{
...
newsize = MM_ALIGN_UP(size SIZEOF_MM_ALLOCNODE);
...
/* Check if this is a request to reduce the size of the allocation. */
oldsize = oldnode->size;
if (newsize <= oldsize)
{
/* Handle the special case where we are not going to change the size
* of the allocation.
*/
if (newsize < oldsize)
{
mm_shrinkchunk(heap, oldnode, newsize);
kasan_poison((FAR char *)oldnode oldnode->size,
oldsize - oldnode->size);
}
/* Then return the original address */
mm_unlock(heap);
MM_ADD_BACKTRACE(heap, oldnode);
return oldmem;
}
...
}其中有个mm_shrinkchunk函数是完成缩小内存块的具体工作的，分为两种情况
- 缩小的内存块后面一块内存是空闲的，此时会减小当前内存块的大小，然后将多余内存合并到后面一块内存中，然后重新插入空闲链表
- 缩小的内存块后面一块内存已经分配，此时减小当前内存块的大小之后，将多余内存生成一个独立的内存块插入到空闲链表中
对应的代码如下：
代码语言：c复制void mm_shrinkchunk(FAR struct mm_heap_s *heap,
FAR struct mm_allocnode_s *node, size_t size)
{
FAR struct mm_freenode_s *next;
DEBUGASSERT((size & MM_GRAN_MASK) == 0);
/* Get a reference to the next node */
// 得到相邻后面一块内存描述符
next = (FAR struct mm_freenode_s *)((FAR char *)node node->size);
/* Check if it is free */
// 后面一块内存空闲
if ((next->preceding & MM_ALLOC_BIT) == 0)
{
FAR struct mm_allocnode_s *andbeyond;
FAR struct mm_freenode_s *newnode;
/* Get the chunk next the next node (which could be the tail chunk) */
andbeyond = (FAR struct mm_allocnode_s *)
((FAR char *)next next->size);
/* Remove the next node. There must be a predecessor, but there may
* not be a successor node.
*/
// 将下一个结点移除
DEBUGASSERT(next->blink);
next->blink->flink = next->flink;
if (next->flink)
{
next->flink->blink = next->blink;
}
/* Create a new chunk that will hold both the next chunk and the
* tailing memory from the aligned chunk.
*/
// 创建一个新的结点
newnode = (FAR struct mm_freenode_s *)((FAR char *)node size);
/* Set up the size of the new node */
// 合并后的大小
newnode->size = next->size node->size - size;
newnode->preceding = size;
node->size = size;
andbeyond->preceding = newnode->size |
(andbeyond->preceding & MM_MASK_BIT);
/* Add the new node to the freenodelist */
// 新结点插入到空闲队列
mm_addfreechunk(heap, newnode);
}
/* The next chunk is allocated. Try to free the end portion at the end
* chunk to be shrunk.
*/
// 后面内存不空闲
else if (node->size >= size SIZEOF_MM_FREENODE)
{
FAR struct mm_freenode_s *newnode;
/* Create a new chunk that will hold both the next chunk and the
* tailing memory from the aligned chunk.
*/
// 创建新结点
newnode = (FAR struct mm_freenode_s *)((FAR char *)node size);
/* Set up the size of the new node */
// 大小为剩余内存大小
newnode->size = node->size - size;
newnode->preceding = size;
node->size = size;
next->preceding = newnode->size |
(next->preceding & MM_MASK_BIT);
/* Add the new node to the freenodelist */
// 插入到空闲队列
mm_addfreechunk(heap, newnode);
}
}
