关于rt-thread内核mem.c程序的疑问

发布于 2010-02-02 15:03:12 浏览：7730 订阅该版

我最近在看rt-thread的动态内存管理函数的实现，好像是从lwip哪里移植过来的，有一个疑问。
```c
void rt_system_heap_init(void* begin_addr, void* end_addr)
{
	struct heap_mem *mem;

/* alignment addr */
	begin_addr = (void*)RT_ALIGN((rt_uint32_t)begin_addr, RT_ALIGN_SIZE);

/* calculate the aligned memory size */
	mem_size_aligned = RT_ALIGN((rt_uint32_t)end_addr - (rt_uint32_t)begin_addr, RT_ALIGN_SIZE) - 2 * sizeof(struct heap_mem);

/* point to begin address of heap */
	heap_ptr = begin_addr;

#ifdef RT_MEM_DEBUG
	rt_kprintf("mem init, heap begin address 0x%x, size %d<br>", (rt_uint32_t)heap_ptr, mem_size_aligned);
#endif

/* initialize the start of the heap */
	mem = (struct heap_mem *)heap_ptr;
	mem->magic= HEAP_MAGIC;
	mem->next = mem_size_aligned;
	mem->prev = 0;
	mem->used = 0;

/* initialize the end of the heap */
	heap_end = (struct heap_mem *)&heap_ptr[mem_size_aligned];
	heap_end->magic= HEAP_MAGIC;
	heap_end->used = 1;
	heap_end->next = mem_size_aligned;
	heap_end->prev = mem_size_aligned;

rt_sem_init(&heap_sem, "heap", 1, RT_IPC_FLAG_FIFO);

/* initialize the lowest-free pointer to the start of the heap */
	lfree = (struct heap_mem *)heap_ptr;
}
```
我的理解是其中`mem->next`是mem的下一个内存管理块基于heap_ptr偏移值，而
```c
mem_size_aligned = RT_ALIGN((rt_uint32_t)end_addr - (rt_uint32_t)begin_addr, RT_ALIGN_SIZE) - 2 * sizeof(struct heap_mem);
```
已经为头尾内存块管理结构留足了空间，所以mem_size_aligned应该是初始内存块实际可用的内存空间，如果`mem->next = mem_size_aligned;`那么初始内存块实际可用的内存只有`mem_size_aligned-SIZEOF_STRUCT_MEM`了，这样不是不合理吗？
所以我觉得应该写成下面这样的
```c
void rt_system_heap_init(void* begin_addr, void* end_addr)
{
	struct heap_mem *mem;

/* alignment addr */
	begin_addr = (void*)RT_ALIGN((rt_uint32_t)begin_addr, RT_ALIGN_SIZE);

/* calculate the aligned memory size */
	mem_size_aligned = RT_ALIGN((rt_uint32_t)end_addr - (rt_uint32_t)begin_addr, RT_ALIGN_SIZE) - 2 * SIZEOF_STRUCT_MEM;

/* point to begin address of heap */
	heap_ptr = begin_addr;

#ifdef RT_MEM_DEBUG
	rt_kprintf("mem init, heap begin address 0x%x, size %d<br>", (rt_uint32_t)heap_ptr, mem_size_aligned);
#endif

/* initialize the start of the heap */
	mem = (struct heap_mem *)heap_ptr;
	mem->magic= HEAP_MAGIC;
	mem->next = mem_size_aligned+SIZEOF_STRUCT_MEM;
	mem->prev = 0;
	mem->used = 0;

/* initialize the end of the heap */
	heap_end = (struct heap_mem *)&heap_ptr[mem->next];
	heap_end->magic= HEAP_MAGIC;
	heap_end->used = 1;
	heap_end->next = mem_size_aligned+SIZEOF_STRUCT_MEM;
	heap_end->prev = mem_size_aligned+SIZEOF_STRUCT_MEM;

rt_sem_init(&heap_sem, "heap", 1, RT_IPC_FLAG_FIFO);

/* initialize the lowest-free pointer to the start of the heap */
	lfree = (struct heap_mem *)heap_ptr;
}
```
同时将mem.c文件中的所有
```c
if (mem2->next != mem_size_aligned)改为if (mem2->next != (mem_size_aligned+SIZEOF_STRUCT_MEM))
```
不知道这样理解对不对，希望哪位高人能帮忙解答一下。