lwip—mem_init和mem_malloc详解

<pre name="code" class="cpp">#define MEM_ALIGNMENT           4//对齐方式为4字节对齐#ifndef LWIP_MEM_ALIGN_SIZE#define LWIP_MEM_ALIGN_SIZE(size) (((size) + MEM_ALIGNMENT - 1) & ~(MEM_ALIGNMENT-1)) //实现待分配数据空间的内存对齐#endif#ifndef LWIP_MEM_ALIGN//地址对齐，对齐方式也为4字节对齐#define LWIP_MEM_ALIGN(addr) ((void *)(((mem_ptr_t)(addr) + MEM_ALIGNMENT - 1) & ~(mem_ptr_t)(MEM_ALIGNMENT-1)))#endif/* MEM_SIZE: the size of the heap memory. If the application will senda lot of data that needs to be copied, this should be set high. */#define MEM_SIZE                (8*1024)//堆的总空间大小，此后在这个基础上划分堆，将在这个空间进行内存分配，内存块结构体和数据都是在这个空间上的//mem为内存块的结构体，next;，prev都为内存块索引struct mem {  /** index (-> ram[next]) of the next struct *///ram为堆的首地址，相当于数组的首地址，索引基地址  mem_size_t next;//next为下一个内存块的索引  /** index (-> ram[next]) of the next struct */  mem_size_t prev;//prev为前一个内存块的索引  /** 1: this area is used; 0: this area is unused */  u8_t used;//标志此内存块已被分配};static struct mem *ram_end;/** All allocated blocks will be MIN_SIZE bytes big, at least! * MIN_SIZE can be overridden to suit your needs. Smaller values save space, * larger values could prevent too small blocks to fragment the RAM too much. */#ifndef MIN_SIZE#define MIN_SIZE             12//内存块大小的最小限制，不能小于12#endif /* MIN_SIZE *//* some alignment macros: we define them here for better source code layout */#define MIN_SIZE_ALIGNED     LWIP_MEM_ALIGN_SIZE(MIN_SIZE)//将MIN_SIZE按4字节对齐，即把12按4字节对齐#define SIZEOF_STRUCT_MEM    LWIP_MEM_ALIGN_SIZE(sizeof(struct mem))//将mem大小按4字节对齐#define MEM_SIZE_ALIGNED     LWIP_MEM_ALIGN_SIZE(MEM_SIZE)//将堆的总空间按4字节对齐，MEM_SIZE在前面，为8*1024//内存对齐解释看我的博文：http://blog.csdn.net/lg2lh/article/details/34853883/** the heap. we need one struct mem at the end and some room for alignment */static u8_t ram_heap[MEM_SIZE_ALIGNED + (2*SIZEOF_STRUCT_MEM) + MEM_ALIGNMENT];//实际开的堆内存空间，MEM_SIZE_ALIGNED为对齐后的数据空间为8192//堆内存的大小为MEM_SIZE_ALIGNED+(2*SIZEOF_STRUCT_MEM)+MEM_ALIGNMENT=8192+2*MEN结构体的大小+4voidmem_init(void){  struct mem *mem;//定义一个mem结构体指针变量  LWIP_ASSERT("Sanity check alignment",    (SIZEOF_STRUCT_MEM & (MEM_ALIGNMENT-1)) == 0);  /* align the heap */  ram = LWIP_MEM_ALIGN(ram_heap);//将堆空间首地址ram_heap按4字节地址对齐  /* initialize the start of the heap */  mem = (struct mem *)ram;//将堆空间ram 首地址强制转换成mem结构体类型，作为首个内存块，但这个内存块还未使用  mem->next = MEM_SIZE_ALIGNED;//把首个内存块的next指针指向了堆空间的最后一个地址（MEM_SIZE_ALIGNED为8*1024），后面实际在mem_malloc时会动态调整next索引,//从而得到实际分配内存空间即为 mem->next减去该内存块mem的地址//待分配内存块的next索引总是指向堆空间最后，好像也不一定，但是按照思路是这样的。  mem->prev = 0;//初始化，因为是第一个内存块，所以前一个内存块不存在，故初始化为0  mem->used = 0;//该内存块没有被分配，待分配状态  /* initialize the end of the heap */  ram_end = (struct mem *)&ram[MEM_SIZE_ALIGNED];//例化一个堆空间末尾内存块，该内存块指向最后一个地址，标志结尾用的已被分配，不可再分配了  ram_end->used = 1;//该内存块已被分配  ram_end->next = MEM_SIZE_ALIGNED;//因为后续再无内存块故，next索引指向最后，即自己  ram_end->prev = MEM_SIZE_ALIGNED;//这个我也不知道啊  mem_sem = sys_sem_new(1);  /* initialize the lowest-free pointer to the start of the heap */  lfree = (struct mem *)ram;//初始化空闲对指针，此时首个内存块是空闲的  MEM_STATS_AVAIL(avail, MEM_SIZE_ALIGNED);}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;  }//size为0的话返回null 分配不成功  /* Expand the size of the allocated memory region so that we can     adjust for alignment. */  size = LWIP_MEM_ALIGN_SIZE(size);//将待分配数据按4字节进行对齐  if(size < MIN_SIZE_ALIGNED) { //如果待分配空间小于MIN_SIZE_ALIGNED（12），则返回分配空间也要为12，最小分配空间为12    /* every data block must be at least MIN_SIZE_ALIGNED long */    size = MIN_SIZE_ALIGNED;  }  if (size > MEM_SIZE_ALIGNED) {//如果待分配空间大于MEM_SIZE_ALIGNED（8*1024），超出堆空间，则返回NULL，无法分配    return NULL;  }  /* protect the heap from concurrent access */  sys_arch_sem_wait(mem_sem, 0);  LWIP_MEM_ALLOC_PROTECT();//未定义#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT  /* run as long as a mem_free disturbed mem_malloc */  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.     *///ptr初值=空闲内存块地址与堆内存首地址之差，如果ptr+size小于堆空间总大小8*1024，则可实现相应大小//的内存块分配，其中ptr实际为已分配了的空间大小，size为待分配的空间大小，两个和一定要小于总空间，才可以实现分配.//判断完成后，将ptr赋值为该内存块next所指地址    for (ptr = (u8_t *)lfree - ram; ptr < MEM_SIZE_ALIGNED - size;         ptr = ((struct mem *)&ram[ptr])->next) {//将待分配的这个内存空间初始化为内存块结构体   mem = (struct mem *)&ram[ptr];#if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT //未定义      mem_free_count = 0;      LWIP_MEM_ALLOC_UNPROTECT();      /* allow mem_free to run */      LWIP_MEM_ALLOC_PROTECT();      if (mem_free_count != 0) {        local_mem_free_count = mem_free_count;      }      mem_free_count = 0;#endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT *///ptr为已分配了的内存空间//后面你会发现，待分配内存块的mem->next始终指向堆空间的最后，即MEM_SIZE_ALIGNED。//内存块未被使用，此时mem为待分配内存块，故mem->next指向MEM_SIZE_ALIGNED，//剩余分配空间（MEM_SIZE_ALIGNED-已分配空间-MEM结构体大小）要大于要待分配空间size      if ((!mem->used) &&          (mem->next - (ptr + SIZEOF_STRUCT_MEM)) >= size) {           /* 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 *///剩余分配空间（MEM_SIZE_ALIGNED-已分配空间-2*MEM结构体大小-12）//要大于要待分配空间size，则才可以进行内存分配。        if (mem->next - (ptr + SIZEOF_STRUCT_MEM) >= (size + SIZEOF_STRUCT_MEM + MIN_SIZE_ALIGNED)) {                 /* (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指向新的待分配内存空间          ptr2 = ptr + SIZEOF_STRUCT_MEM + size;           /* create mem2 struct *///mem2为新的待分配内存块结构体          mem2 = (struct mem *)&ram[ptr2]; //新的内存块mem2未被使用          mem2->used = 0;//新的待分配的内存块mem2的next索引指向堆空间的最后，即MEM_SIZE_ALIGNED           mem2->next = mem->next;//而新的内存块的prev索引是我们这次正在分配的模块索引，即ptr           mem2->prev = ptr;          /* and insert it between mem and mem->next *///把本次分配的mem内存块的next索引重新定位，指向新的待分配的模块的索引，不再指向堆空间最后          mem->next = ptr2;          mem->used = 1;//本内存块被使用//我之前分析的都是新的待分配内存块next索引应该始终指向堆空间最后的，这里竟然判断了，可能存在不指向最后的情况//具体原因还没分析。如果新的待分配内存块mem2的next索引未指向最后，则需要将它所指向的索引内存块的prev索引指向 //他自己ptr2。          if (mem2->next != MEM_SIZE_ALIGNED) {            ((struct mem *)&ram[mem2->next])->prev = ptr2;          }          MEM_STATS_INC_USED(used, (size + SIZEOF_STRUCT_MEM));        } else {//如果没有满足对应if条件，则直接分配完改内存块即可，也不用指向下一个待分配的内存块，因为没有空间可以再分配了          /* (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 - ((u8_t *)mem - ram));        }        if (mem == lfree) {//将空闲指针索引指向新的待分配内存块索引ram[lfree->next]，即ptr2          /* Find next free block after mem and update lowest free pointer */          while (lfree->used && lfree != ram_end) {            LWIP_MEM_ALLOC_UNPROTECT();            /* prevent high interrupt latency... */            LWIP_MEM_ALLOC_PROTECT();            lfree = (struct mem *)&ram[lfree->next];          }          LWIP_ASSERT("mem_malloc: !lfree->used", ((lfree == ram_end) || (!lfree->used)));        }        LWIP_MEM_ALLOC_UNPROTECT();        sys_sem_signal(mem_sem);        LWIP_ASSERT("mem_malloc: allocated memory not above ram_end.",         (mem_ptr_t)mem + SIZEOF_STRUCT_MEM + size <= (mem_ptr_t)ram_end);        LWIP_ASSERT("mem_malloc: allocated memory properly aligned.",         ((mem_ptr_t)mem + SIZEOF_STRUCT_MEM) % MEM_ALIGNMENT == 0);        LWIP_ASSERT("mem_malloc: sanity check alignment",          (((mem_ptr_t)mem) & (MEM_ALIGNMENT-1)) == 0);        return (u8_t *)mem + SIZEOF_STRUCT_MEM;//返回分配结果，即已分配内存块数据空间的首地址。      }    }#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 | 2, ("mem_malloc: could not allocate %"S16_F" bytes\n", (s16_t)size));  MEM_STATS_INC(err);  LWIP_MEM_ALLOC_UNPROTECT();  sys_sem_signal(mem_sem);  return NULL;}