[RT-Thread 源码分析] 2. 内存管理2

/**
* This function will change the previously allocated memory block.
*
* @param rmem pointer to memory allocated by rt_malloc
* @param newsize the required new size
*
* @return the changed memory block address
*/
void *rt_realloc(void *rmem, rt_size_t newsize)
{
rt_size_t size;
rt_size_t ptr, ptr2;
struct heap_mem *mem, *mem2;
void *nmem;

RT_DEBUG_NOT_IN_INTERRUPT;

/* alignment size */
// 首先要将地址对齐，然后计算新分配的大小
newsize = RT_ALIGN(newsize, RT_ALIGN_SIZE);
if (newsize > mem_size_aligned)
{
// 新分配的内存大小不能大于总的大小
RT_DEBUG_LOG(RT_DEBUG_MEM, ("realloc: out of memory\n"));

return RT_NULL;
}

/* allocate a new memory block */
// 如果传入的指针为空，则直接调用malloc分配一个新的内存块
if (rmem == RT_NULL)
return rt_malloc(newsize);
// 等待信号量，防止内存分配竞争
rt_sem_take(&heap_sem, RT_WAITING_FOREVER);

if ((rt_uint8_t *)rmem < (rt_uint8_t *)heap_ptr ||
(rt_uint8_t *)rmem >= (rt_uint8_t *)heap_end)
{
// 如果传入的内存指针小于最小值和大于最大值，则为非法
// 然后释放信号量，退出。
/* illegal memory */
rt_sem_release(&heap_sem);

return rmem;
}

mem = (struct heap_mem *)((rt_uint8_t *)rmem - SIZEOF_STRUCT_MEM);
// 因为传入的内存块的地址是不包括内存控制块的，这里需要重新加上

ptr = (rt_uint8_t *)mem - heap_ptr;         // 得到相对偏移量
size = mem->next - ptr - SIZEOF_STRUCT_MEM; // 得到当前这个内存块的大小
if (size == newsize)
{
/* the size is the same as */
// 如果大小没变，则不用再次分配，直接返回
rt_sem_release(&heap_sem);

return rmem;
}

if (newsize + SIZEOF_STRUCT_MEM + MIN_SIZE < size)  //新大小满足要求
{
/* split memory block */
#ifdef RT_MEM_STATS
used_mem -= (size - newsize);
#endif
// 这一段就是把原来的ptr扩大
// ptr2是指向后一个未用的块
ptr2 = ptr + SIZEOF_STRUCT_MEM + newsize;   // 指向新的内存块
mem2 = (struct heap_mem *)&heap_ptr[ptr2];  // 转换为内存控制块
mem2->magic= HEAP_MAGIC;
mem2->used = 0;
mem2->next = mem->next;
mem2->prev = ptr;
mem->next = ptr2;                           // 链表操作，不再重复
if (mem2->next != mem_size_aligned + SIZEOF_STRUCT_MEM)
{
// 如果不是这个heap的末尾，就让mem2的后一个块的prev指向mem2
((struct heap_mem *)&heap_ptr[mem2->next])->prev = ptr2;
}
// 这个函数处理一些零碎的空间
plug_holes(mem2);

rt_sem_release(&heap_sem);

return rmem;
}
rt_sem_release(&heap_sem);

/* expand memory */
// 如果新大小不满足要求，则直接开辟一块内存
nmem = rt_malloc(newsize);
if (nmem != RT_NULL) /* check memory */
{
rt_memcpy(nmem, rmem, size < newsize ? size : newsize);
// 将原来内存块的内容拷贝到新内存块中
rt_free(rmem);
}

return nmem;
}
RTM_EXPORT(rt_realloc);

/**
* This function will contiguously allocate 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 *rt_calloc(rt_size_t count, rt_size_t size)
{
void *p;

RT_DEBUG_NOT_IN_INTERRUPT;

/* allocate 'count' objects of size 'size' */
// 使用malloc分配
p = rt_malloc(count * size);

/* zero the memory */
// 将内存中元素全部置零
if (p)
rt_memset(p, 0, count * size);

return p;
}
RTM_EXPORT(rt_calloc);

/**
* This function will release the previously allocated memory block by
* rt_malloc. The released memory block is taken back to system heap.
*
* @param rmem the address of memory which will be released
*/
void rt_free(void *rmem)
{
struct heap_mem *mem;

RT_DEBUG_NOT_IN_INTERRUPT;

if (rmem == RT_NULL)
return;
RT_ASSERT((((rt_uint32_t)rmem) & (RT_ALIGN_SIZE-1)) == 0);
RT_ASSERT((rt_uint8_t *)rmem >= (rt_uint8_t *)heap_ptr &&
(rt_uint8_t *)rmem < (rt_uint8_t *)heap_end);

RT_OBJECT_HOOK_CALL(rt_free_hook, (rmem));

if ((rt_uint8_t *)rmem < (rt_uint8_t *)heap_ptr ||
(rt_uint8_t *)rmem >= (rt_uint8_t *)heap_end)
// 确保传入内存地址合法
{
RT_DEBUG_LOG(RT_DEBUG_MEM, ("illegal memory\n"));

return;
}

/* Get the corresponding struct heap_mem ... */
// 获得传入内存块的内存控制块
// 因为内存控制块在内存块的前部
mem = (struct heap_mem *)((rt_uint8_t *)rmem - SIZEOF_STRUCT_MEM);

RT_DEBUG_LOG(RT_DEBUG_MEM,
("release memory 0x%x, size: %d\n",
(rt_uint32_t)rmem,
(rt_uint32_t)(mem->next - ((rt_uint8_t *)mem - heap_ptr))));

/* protect the heap from concurrent access */
rt_sem_take(&heap_sem, RT_WAITING_FOREVER);

/* ... which has to be in a used state ... */
RT_ASSERT(mem->used);
RT_ASSERT(mem->magic == HEAP_MAGIC);
/* ... and is now unused. */
// 置位成不用状态
mem->used  = 0;
mem->magic = 0;

if (mem < lfree)
{
/* the newly freed struct is now the lowest */
// lfree指向最低可用内存块控制块地址
// 这里更新lfree
lfree = mem;
}

#ifdef RT_MEM_STATS
used_mem -= (mem->next - ((rt_uint8_t*)mem - heap_ptr));
#endif

/* finally, see if prev or next are free also */
// 处理零碎的块
plug_holes(mem);
rt_sem_release(&heap_sem);
}

static void plug_holes(struct heap_mem *mem)
{
struct heap_mem *nmem;
struct heap_mem *pmem;

RT_ASSERT((rt_uint8_t *)mem >= heap_ptr);
RT_ASSERT((rt_uint8_t *)mem < (rt_uint8_t *)heap_end);
RT_ASSERT(mem->used == 0);

/* plug hole forward */
// 把后面一个内存空洞填满
// 获取后一个内存控制块
nmem = (struct heap_mem *)&heap_ptr[mem->next];
if (mem != nmem &&
nmem->used == 0 &&
(rt_uint8_t *)nmem != (rt_uint8_t *)heap_end)
{
/* if mem->next is unused and not end of heap_ptr,
* combine mem and mem->next
*/
// 如果存在后一个内存块，且没有使用，且不是末尾
if (lfree == nmem)          // 如果后一个是lfree
{
lfree = mem;            // 更新lfree为mem
}
mem->next = nmem->next;     // 链表操作，即删除nmem
((struct heap_mem *)&heap_ptr[nmem->next])->prev = (rt_uint8_t *)mem - heap_ptr;
// 为什么这么烦？
// 因为1. next, prev都是使用相对地址
// 2. heap_ptr和mem的指针类型不一样
}

/* plug hole backward */
// 把前一个空洞填满
// 获取前一个内存控制块
pmem = (struct heap_mem *)&heap_ptr[mem->prev];
if (pmem != mem && pmem->used == 0)
{
/* if mem->prev is unused, combine mem and mem->prev */
if (lfree == mem)
{
lfree = pmem;   // 更新lfree
}
pmem->next = mem->next;
((struct heap_mem *)&heap_ptr[mem->next])->prev = (rt_uint8_t *)pmem - heap_ptr;
// 链表操作，删除
}
}