lwip-mem_init和mem_malloc详解

lwip-mem_init和mem_malloc详解

[cpp] view plain copy <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 send a 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：

https://www.360docs.net/doc/0f7213784.html,/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+4 void mem_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_AV AIL(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; }