1.1 动态内存池 1.1.1 原理 此种方式下，用户只能申请固定大小的空间，例如UDP控制块，TCP控制块。 1.1.2 Lwip的实现源码 下述代码主要用于内存池的初始化，Lwip协议栈中，把所有的pool挨个放在一起，并把它们放在一片连续的内存空间，这样形成了一个巨大的内存池。
void memp_init(void) { u16_t i; /* for every pool: */ for (i = 0; i < LWIP_ARRAYSIZE(memp_pools); i++) { memp_init_pool(memp_pools[i]); #if LWIP_STATS && MEMP_STATS lwip_stats.memp[i] = memp_pools[i]->stats; #endif } #if MEMP_OVERFLOW_CHECK >= 2 /* check everything a first time to see if it worked */ memp_overflow_check_all(); #endif /* MEMP_OVERFLOW_CHECK >= 2 */ }上述代码主要用于对各个内存池的初始memp_pools，那么究竟是怎么初始化的呢?如下述代码所描述的那样，删除了代码中预编译部分。
void memp_init_pool(const struct memp_desc *desc) { int i; struct memp *memp; *desc->tab = NULL;//初始为空指针 memp = (struct memp *)LWIP_MEM_ALIGN(desc->base);//将内存池对齐 /* create a linked list of memp elements */ for (i = 0; i < desc->num; ++i) {//依次对每种类型的POOL进行操作 memp->next = *desc->tab;//将所有pool组成链表 *desc->tab = memp; /* cast through void* to get rid of alignment warnings */ memp = (struct memp *)(void *)((u8_t *)memp + MEMP_SIZE + desc->size } }1.2 动态内存堆栈 1.2.1 原理 动态内存堆分配策略本质上是对一个事先定义好的内存块进行合理有效的组织和管理，其内存分配的策略采用首次拟合方式，只要找到一个比用户空间请求空间大的空闲块，从中切割出合适的块，把剩余的部分返回到动态内存堆中。 在这种策略下，用户申请内存块大小具有最小限制，即请求的大小不能小于MIN_SIZE,否则系统自动请求大小设置为MIN_SIZE。通常用户可以自定义该值达到节省内存空间的目的，不过会导致大的内存块不断被分成小的内存块，内存释放时的过程则相反，内存函数会查看该节点前后的内存块是否空闲，如果空闲则合成一个大的内存空闲块。 采用这种方式其优点时内存浪费小，比较简单，适合小内存。其缺点是如果频繁的动态内存分配和释放，可能会造成严重的内存碎片，如果在内存碎片严重的情况下，可能会导致内存分配不成功
1.2.2 源码分析 （1）初始化
void mem_init(void) { struct mem *mem; LWIP_ASSERT("Sanity check alignment", (SIZEOF_STRUCT_MEM & (MEM_ALIGNMENT - 1)) == 0); /* align the heap */ //内存堆栈对齐，ram 用以记录对齐后的起始地址 ram = (u8_t *)LWIP_MEM_ALIGN(LWIP_RAM_HEAP_POINTER); /* initialize the start of the heap */ mem = (struct mem *)(void *)ram;//在起始地址放置一个mem结构体 mem->next = MEM_SIZE_ALIGNED;//下一个偏移量 mem->prev = 0;//上一内存块为空 mem->used = 0;//下一内存块为空 /* initialize the end of the heap */ ram_end = ptr_to_mem(MEM_SIZE_ALIGNED); ram_end->used = 1;//标记为已用 ram_end->next = MEM_SIZE_ALIGNED;//下已内存块，指向自身 ram_end->prev = MEM_SIZE_ALIGNED;//下已内存块，指向自身 MEM_SANITY(); /* initialize the lowest-free pointer to the start of the heap */ lfree = (struct mem *)(void *)ram; MEM_STATS_AVAIL(avail, MEM_SIZE_ALIGNED);//获取内存状态 if (sys_mutex_new(&mem_mutex) != ERR_OK) { LWIP_ASSERT("failed to create mem_mutex", 0); } }（2）内存申请
void *mem_malloc(mem_size_t size_in) { mem_size_t ptr, ptr2, size; 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_in == 0) { return NULL; } /* Expand the size of the allocated memory region so that we can adjust for alignment. */ size = (mem_size_t)LWIP_MEM_ALIGN_SIZE(size_in); if (size < MIN_SIZE_ALIGNED) { /* every data block must be at least MIN_SIZE_ALIGNED long */ size = MIN_SIZE_ALIGNED; } #if MEM_OVERFLOW_CHECK size += MEM_SANITY_REGION_BEFORE_ALIGNED + MEM_SANITY_REGION_AFTER_ALIGNED; #endif if ((size > MEM_SIZE_ALIGNED) || (size < size_in)) { return NULL; } /* protect the heap from concurrent access */ sys_mutex_lock(&mem_mutex); LWIP_MEM_ALLOC_PROTECT(); #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT /* run as long as a mem_free disturbed mem_malloc or mem_trim */ 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. */ for (ptr = mem_to_ptr(lfree); ptr < MEM_SIZE_ALIGNED - size; ptr = ptr_to_mem(ptr)->next) { mem = ptr_to_mem(ptr); #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT mem_free_count = 0; LWIP_MEM_ALLOC_UNPROTECT(); /* allow mem_free or mem_trim to run */ LWIP_MEM_ALLOC_PROTECT(); if (mem_free_count != 0) { /* If mem_free or mem_trim have run, we have to restart since they could have altered our current struct mem. */ local_mem_free_count = 1; break; } #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */ 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 */ 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 = (mem_size_t)(ptr + SIZEOF_STRUCT_MEM + size); LWIP_ASSERT("invalid next ptr",ptr2 != MEM_SIZE_ALIGNED); /* create mem2 struct */ mem2 = ptr_to_mem(ptr2); mem2->used = 0; mem2->next = mem->next; mem2->prev = ptr; /* and insert it between mem and mem->next */ mem->next = ptr2; mem->used = 1; if (mem2->next != MEM_SIZE_ALIGNED) { ptr_to_mem(mem2->next)->prev = ptr2; } MEM_STATS_INC_USED(used, (size + SIZEOF_STRUCT_MEM)); } else { /* (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 exact 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 - mem_to_ptr(mem)); } #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT mem_malloc_adjust_lfree: #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */ if (mem == lfree) { struct mem *cur = lfree; /* Find next free block after mem and update lowest free pointer */ while (cur->used && cur != ram_end) { #if LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT mem_free_count = 0; LWIP_MEM_ALLOC_UNPROTECT(); /* prevent high interrupt latency... */ LWIP_MEM_ALLOC_PROTECT(); if (mem_free_count != 0) { /* If mem_free or mem_trim have run, we have to restart since they could have altered our current struct mem or lfree. */ goto mem_malloc_adjust_lfree; } #endif /* LWIP_ALLOW_MEM_FREE_FROM_OTHER_CONTEXT */ cur = ptr_to_mem(cur->next); } lfree = cur; LWIP_ASSERT("mem_malloc: !lfree->used", ((lfree == ram_end) || (!lfree->used))); } LWIP_MEM_ALLOC_UNPROTECT(); sys_mutex_unlock(&mem_mutex); 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); #if MEM_OVERFLOW_CHECK mem_overflow_init_element(mem, size_in); #endif MEM_SANITY(); return (u8_t *)mem + SIZEOF_STRUCT_MEM + MEM_SANITY_OFFSET; } } #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 */ MEM_STATS_INC(err); LWIP_MEM_ALLOC_UNPROTECT(); sys_mutex_unlock(&mem_mutex); LWIP_DEBUGF(MEM_DEBUG | LWIP_DBG_LEVEL_SERIOUS, ("mem_malloc: could not allocate %"S16_F" bytes\n", (s16_t)size)); return NULL; }