lwip之mem

mem概述、mem的作用

在LwIP中，内存管理部分由mem.h、mem.c中实现，支持多种分配策略，比较灵活。支持运行时库自带的内存分配(MEM_LIBC_MALLOC)，内存池分配(MEM_USE_POOLS)，动态内存堆分配，这些分配策略可以通过宏定义来更改。在嵌入式系统里面，C运行时库自带的内存分配一般情况下很少用，更多的是后面二者。

mem的结构（分析使用pool的情况）

struct mem { 
/** index (-> ram[next]) of the next struct */ 
mem_size_t next; 
/** index (-> ram[next]) of the next struct */ 
mem_size_t prev; 
/** 1: this area is used; 0: this area is unused */ 
u8_t used; 
};

图示

接口函数

/** 
* Zero the heap and initialize start, end and lowest-free 
*/ 
void mem_init(void);

/** 
* Put a struct mem back on the heap 
* 
* @param rmem is the data portion of a struct mem as returned by a previous 
* call to mem_malloc() 
*/ 
void mem_free(void *rmem)

mem源码分析

全局变量

/** 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]; 
/** pointer to the heap (ram_heap): for alignment, ram is now a pointer instead of an array */ 
static u8_t *ram; 
/** the last entry, always unused! */ 
static struct mem *ram_end; 
/** pointer to the lowest free block, this is used for faster search */ 
static struct mem *lfree;

lwip初始化时在内存中开辟ram供使用！可以使用的内存大小为MEM_SIZE，在每一块已分配的内存块的顶端都有一个这样的结构体，所有内存块被链接为双向链表，全局变量struct mem*lfree指向最后一个未使用的mem.

void mem_init(void) 
{ 
struct mem *mem;

LWIP_ASSERT("Sanity check alignment", (SIZEOF_STRUCT_MEM & (MEM_ALIGNMENT-1)) == 0);

/* align the heap */ 
ram = LWIP_MEM_ALIGN(ram_heap); 
/* initialize the start of the heap */ 
mem = (struct mem *)ram; //ram为全局变量

//next和prev并不是指针，他们只是代表下一个（前一个）可使用的内存块大小

mem->next = MEM_SIZE_ALIGNED; 
mem->prev = 0; 
mem->used = 0; 
/* initialize the end of the heap */ 
ram_end = (struct mem *)&ram[MEM_SIZE_ALIGNED]; 
ram_end->used = 1;//不能再分了 
ram_end->next = MEM_SIZE_ALIGNED; 
ram_end->prev = MEM_SIZE_ALIGNED;

mem_sem = sys_sem_new(1);//申请信号量，同步用
/* initialize the lowest-free pointer to the start of the heap */ 
lfree = (struct mem *)ram;

MEM_STATS_AVAIL(avail, MEM_SIZE_ALIGNED); 
}

总结