[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
    }
}