C语言malloc申请内存时的碎片问题

解决问题：malloc在申请内存的时候，内存碎片问题会导致原本内存大小足够，却申请大内存失败；

比如：原本内存还有10M内存，此时先申请4M内存，再申请16Bytes内存，之后把4M内存释放掉，按理来说，此时应该还有 10M - 16Bytes 内存，但此时，再去申请8M的大内存，则申请失败。

因为malloc申请的内存，必须是一块连续的内存，但此时中间已经有16Bytes内存碎片导致内存不连续，所以申请内存失败；

以下是我针对碎片问题，对内存管理机制做出一种优化方案：在开机初始化内存之后，先申请一块1M左右内存（根据情况修改大小），用作内存碎片管理，然后把这1M内存分为很多个小内存，并把小内存的地址放在链接节点中，之后申请内存时，优先判断内存碎片管理中是否有满足大小的小内存。

有的话，直接使用提前申请的小内存就可以了，如果内存管理机制中没有适合的内存，但重新用malloc()函数申请；

接下来，解释我写的碎片管理机制：

1 mm_management_init()初始化

 

void mm_management_init(unsigned int free_memory_start, unsigned int free_memory_end)

 

传入参数free_memory_start是内存初始化之后，剩余可申请的首地址，该地址，一般会传入到main函数，如果main()函数没有传入该参数的话，可以在内存初始化之后，自己malloc(4)申请一下，把返回的地址作为mm_management_init()函数的第一个参数；

传入参数free_memory_end是可以申请的最大地址，每个IC各有不同；

mm_management_init()对16bytes，64bytes，256bytes，512bytes，1024bytes，4096bytes这些小内存做优化，提前计算小内存占用的总大小。

然后直接申请这块大内存占住，再把这块大内存分配给各个小内存，并记录在链表中，比如：mm_fix_16_head

2 mm_management_malloc()申请函

 

unsigned int  mm_management_malloc(unsigned int size)

   
 申请内存的时候，先判断size大小，如果大小可以在内存管理机制中找到，则直接返回提前申请地址，如果大小不满足，或者小内存已被申请完，则用malloc重新申请。    
 在内存管理机制中拿到的小内存，该链表节点的标记会设为MM_STATUS_BUSY。

 

3 mm_management_free()

 

void mm_management_free(void *mm_ptr)

  
  与mm_management_malloc()相反，先检查所有小内存链表是都有该地址，有的话就把该地址内存清0，并把标记设为MM_STATUS_FREE；如果是用malloc申请的，当时是free()释放掉；

 

 
   接下来是代码：

 

#include
#include


#define C_MM_16BYTE_NUM    (32)
#define C_MM_64BYTE_NUM    (16)
#define C_MM_256BYTE_NUM   (12)
#define C_MM_512BYTE_NUM   (12)
#define C_MM_1024BYTE_NUM   (18)
#define C_MM_4096BYTE_NUM   (30)


#define C_MM_16BYTE     (16)
#define C_MM_64BYTE     (64)
#define C_MM_256BYTE    (256)
#define C_MM_512BYTE    (512)
#define C_MM_1024BYTE    (1024)
#define C_MM_4096BYTE    (4096)


#define C_MM_MAX_SIZE    C_MM_4096BYTE //碎片管理最大的碎片大小


#define MM_STATUS_FREE    (0)   //0:表示内存空闲
#define MM_STATUS_BUSY    (1)   //1:表示内存已被申请


#define MM_STATUS_OK                (0)
#define MM_STATUS_FAIL              (1)


typedef struct mm_node_struct {
 unsigned int    *mm_node;   //存放内存节点指针
 unsigned short   iflag;    //指针是否空闲
 struct P_MM_Node_STRUCT *next;    //指向下一个内存节点指针
} MM_Node_STRUCT, *P_MM_Node_STRUCT;


typedef struct mm_sdram_struct {
 unsigned int    count;
 P_MM_Node_STRUCT  *next;
} MM_SDRAM_STRUCT, *P_MM_SDRAM_STRUCT;


static MM_SDRAM_STRUCT mm_fix_16_head;
static MM_SDRAM_STRUCT mm_fix_64_head;
static MM_SDRAM_STRUCT mm_fix_256_head;
static MM_SDRAM_STRUCT mm_fix_512_head;
static MM_SDRAM_STRUCT mm_fix_1024_head;
static MM_SDRAM_STRUCT mm_fix_4096_head;


static P_MM_SDRAM_STRUCT pmm_fix_16_head = &mm_fix_16_head;
static P_MM_SDRAM_STRUCT pmm_fix_64_head = &mm_fix_64_head;
static P_MM_SDRAM_STRUCT pmm_fix_256_head = &mm_fix_256_head;
static P_MM_SDRAM_STRUCT pmm_fix_512_head = &mm_fix_512_head;
static P_MM_SDRAM_STRUCT pmm_fix_1024_head = &mm_fix_1024_head;
static P_MM_SDRAM_STRUCT pmm_fix_4096_head = &mm_fix_4096_head;


static P_MM_Node_STRUCT mm_management_getnode(P_MM_SDRAM_STRUCT pmm_fix_head);
static unsigned int mm_management_node_free(P_MM_SDRAM_STRUCT pmm_fix_head, unsigned int *mm_ptr, unsigned int size);


static unsigned int *mm_management_ptr = NULL;
static unsigned int mm_management_size = 0;


/*
** free_memory_start : 开机内存初始化之后，剩余可以申请的地址的首地址
** free_memory_end   : 内存可以申请的最大地址
*/
void mm_management_init(unsigned int free_memory_start, unsigned int free_memory_end)
{
 unsigned int mm_usesize=0,offset=0,mm_offset;
 unsigned char *ptr_tmp;
 unsigned int i;
 P_MM_Node_STRUCT pmm_fix_head, pmm_fix_tmp;


 free_memory_start = (free_memory_start + 3) & (~0x3);  // Align to 4-bytes boundary
 free_memory_end   = (free_memory_end + 3) & (~0x3);   // Align to 4-bytes boundary


 do{
  //[1]判断剩余内存是否满足碎片管理所需大小
  mm_usesize = 0;
  mm_usesize += C_MM_16BYTE * C_MM_16BYTE_NUM;
  mm_usesize += C_MM_64BYTE * C_MM_64BYTE_NUM;
  mm_usesize += C_MM_256BYTE * C_MM_256BYTE_NUM;
  mm_usesize += C_MM_512BYTE * C_MM_512BYTE_NUM;
  mm_usesize += C_MM_1024BYTE * C_MM_1024BYTE_NUM;
  mm_usesize += C_MM_4096BYTE * C_MM_4096BYTE_NUM;


  if(mm_usesize+free_memory_start > free_memory_end)
  {
   printf("free memory not enough for mm management,init fail
");
   break;
  }
  mm_management_ptr = (unsigned char *)malloc(mm_usesize);    //申请整块碎片管理内存大小  //如果有malloc_align函数，建议改用malloc_align申请64bit对其的内存
  if(mm_management_ptr == NULL)
  {
   printf("mm management malloc fail,init fail
");
   break;
  }
  mm_management_size = mm_usesize;
  ptr_tmp = mm_management_ptr;
  memset(ptr_tmp, 0x00, mm_usesize);


  //[2]内存链表头初始化，用于存放以下步骤的子链表节点
  memset((void*)pmm_fix_16_head, 0x00, sizeof(mm_fix_16_head));
  memset((void*)pmm_fix_64_head, 0x00, sizeof(mm_fix_64_head));
  memset((void*)pmm_fix_256_head, 0x00, sizeof(mm_fix_256_head));
  memset((void*)pmm_fix_512_head, 0x00, sizeof(mm_fix_512_head));
  memset((void*)pmm_fix_1024_head, 0x00, sizeof(mm_fix_1024_head));
  memset((void*)pmm_fix_4096_head, 0x00, sizeof(mm_fix_4096_head));  
  
  //[3]申请16Bytes碎片内存存放在链表  
  mm_offset = 0;
  mm_fix_16_head.count = C_MM_16BYTE_NUM;
  pmm_fix_head = pmm_fix_16_head;
  for(i=0; iiflag = MM_STATUS_FREE;
   pmm_fix_tmp->next = NULL;
   offset = (C_MM_16BYTE * i) + mm_offset;    //计算小内存碎片在大buf里的偏移地址
   pmm_fix_tmp->mm_node = ptr_tmp + offset;
   
   pmm_fix_head->next = pmm_fix_tmp;
   pmm_fix_head = pmm_fix_tmp;
  }


  //[4]申请64Bytes碎片内存存放在链表  
  mm_offset += C_MM_16BYTE * C_MM_16BYTE_NUM;
  mm_fix_64_head.count = C_MM_64BYTE_NUM;
  pmm_fix_head = pmm_fix_64_head;
  for(i=0; iiflag = MM_STATUS_FREE;
   pmm_fix_tmp->next = NULL;
   offset = (C_MM_64BYTE * i) + mm_offset;    //计算小内存碎片在大buf里的偏移地址
   pmm_fix_tmp->mm_node = ptr_tmp + offset;
   
   pmm_fix_head->next = pmm_fix_tmp;
   pmm_fix_head = pmm_fix_tmp;
  }


  //[5]申请256Bytes碎片内存存放在链表  
  mm_offset += C_MM_64BYTE * C_MM_64BYTE_NUM;
  mm_fix_256_head.count = C_MM_256BYTE_NUM;
  pmm_fix_head = pmm_fix_256_head;
  for(i=0; iiflag = MM_STATUS_FREE;
   pmm_fix_tmp->next = NULL;
   offset = (C_MM_256BYTE * i) + mm_offset;   //计算小内存碎片在大buf里的偏移地址
   pmm_fix_tmp->mm_node = ptr_tmp + offset;
   
   pmm_fix_head->next = pmm_fix_tmp;
   pmm_fix_head = pmm_fix_tmp;
  }


  //[6]申请512Bytes碎片内存存放在链表  
  mm_offset += C_MM_256BYTE * C_MM_256BYTE_NUM;
  mm_fix_512_head.count = C_MM_512BYTE_NUM;
  pmm_fix_head = pmm_fix_512_head;
  for(i=0; iiflag = MM_STATUS_FREE;
   pmm_fix_tmp->next = NULL;
   offset = (C_MM_512BYTE * i) + mm_offset;   //计算小内存碎片在大buf里的偏移地址
   pmm_fix_tmp->mm_node = ptr_tmp + offset;
   
   pmm_fix_head->next = pmm_fix_tmp;
   pmm_fix_head = pmm_fix_tmp;
  }


  //[7]申请1024Bytes碎片内存存放在链表  
  mm_offset += C_MM_512BYTE * C_MM_512BYTE_NUM;
  mm_fix_1024_head.count = C_MM_1024BYTE_NUM;
  pmm_fix_head = pmm_fix_1024_head;
  for(i=0; iiflag = MM_STATUS_FREE;
   pmm_fix_tmp->next = NULL;
   offset = (C_MM_1024BYTE * i) + mm_offset;   //计算小内存碎片在大buf里的偏移地址
   pmm_fix_tmp->mm_node = ptr_tmp + offset;
   
   pmm_fix_head->next = pmm_fix_tmp;
   pmm_fix_head = pmm_fix_tmp;
  }
  
  //[8]申请4096Bytes碎片内存存放在链表  
  mm_offset += C_MM_1024BYTE * C_MM_1024BYTE_NUM;
  mm_fix_4096_head.count = C_MM_4096BYTE_NUM;
  pmm_fix_head = pmm_fix_4096_head;
  for(i=0; iiflag = MM_STATUS_FREE;
   pmm_fix_tmp->next = NULL;
   offset = (C_MM_4096BYTE * i) + mm_offset;   //计算小内存碎片在大buf里的偏移地址
   pmm_fix_tmp->mm_node = ptr_tmp + offset;
   
   pmm_fix_head->next = pmm_fix_tmp;
   pmm_fix_head = pmm_fix_tmp;
  }
 }while(0);
 
 printf("mm management init end!!!
");
}


unsigned int  mm_management_malloc(unsigned int size)
{
 int status = MM_STATUS_FAIL;   //MM_STATUS_FAIL表示还没申请到碎片内存
 P_MM_Node_STRUCT  pmm_fix_node;
 unsigned int *mm_ptr = NULL;


 //获取空闲碎片节点
 do{


  //[1]判断申请内存大小是否满足要求
  if(size < 0)
  {
   status = MM_STATUS_FAIL;
   printf("mm management malloc size is error
");
   return NULL;
  }


  //[2]判断大小是否小于16Byets
  if(size < C_MM_16BYTE && status == MM_STATUS_FAIL)
  {
   pmm_fix_node = mm_management_getnode(pmm_fix_16_head);
   if(pmm_fix_node != NULL)
   {
    status = MM_STATUS_OK;
    break;
   }
  }


  //[3]判断大小是否小于64Byets
  if(size < C_MM_64BYTE && status == MM_STATUS_FAIL)
  {
   pmm_fix_node = mm_management_getnode(pmm_fix_64_head);
   if(pmm_fix_node != NULL)
   {
    status = MM_STATUS_OK;
    break;
   }
  }


  //[4]判断大小是否小于256Byets
  if(size < C_MM_256BYTE && status == MM_STATUS_FAIL)
  {
   pmm_fix_node = mm_management_getnode(pmm_fix_256_head);
   if(pmm_fix_node != NULL)
   {
    status = MM_STATUS_OK;
    break;
   }
  }


  //[5]判断大小是否小于512Byets
  if(size < C_MM_512BYTE && status == MM_STATUS_FAIL)
  {
   pmm_fix_node = mm_management_getnode(pmm_fix_512_head);
   if(pmm_fix_node != NULL)
   {
    status = MM_STATUS_OK;
    break;
   }
  }


  //[6]判断大小是否小于1024Byets
  if(size < C_MM_1024BYTE && status == MM_STATUS_FAIL)
  {
   pmm_fix_node = mm_management_getnode(pmm_fix_1024_head);
   if(pmm_fix_node != NULL)
   {
    status = MM_STATUS_OK;
    break;
   }
  }


  //[7]判断大小是否小于4096Byets
  if(size < C_MM_4096BYTE && status == MM_STATUS_FAIL)
  {
   pmm_fix_node = mm_management_getnode(pmm_fix_4096_head);
   if(pmm_fix_node != NULL)
   {
    status = MM_STATUS_OK;
    break;
   }
  }
 }while(0);


 if(status == MM_STATUS_OK)


 {
  mm_ptr = pmm_fix_node->mm_node;
  pmm_fix_node->iflag = MM_STATUS_BUSY;
 }
 else
 {
  mm_ptr = (unsigned int *)malloc(size);
 }


 return (unsigned int *)mm_ptr;
}


void mm_management_free(void *mm_ptr)
{
 unsigned int i;
 int status = MM_STATUS_FAIL;
 P_MM_Node_STRUCT  pmm_fix_node;


 do{
  //[1]如果地址是16Bytes碎片地址，则释放内存
  status = mm_management_node_free(pmm_fix_16_head, mm_ptr, C_MM_16BYTE);
  if(status == MM_STATUS_OK)
   break;


  //[2]如果地址是64Bytes碎片地址，则释放内存
  status = mm_management_node_free(pmm_fix_64_head, mm_ptr, C_MM_64BYTE);
  if(status == MM_STATUS_OK)
   break;


  //[1]如果地址是256Bytes碎片地址，则释放内存
  status = mm_management_node_free(pmm_fix_256_head, mm_ptr, C_MM_256BYTE);
  if(status == MM_STATUS_OK)
   break;


  //[1]如果地址是512Bytes碎片地址，则释放内存
  status = mm_management_node_free(pmm_fix_512_head, mm_ptr, C_MM_512BYTE);
  if(status == MM_STATUS_OK)
   break;


  //[1]如果地址是1024Bytes碎片地址，则释放内存
  status = mm_management_node_free(pmm_fix_1024_head, mm_ptr, C_MM_1024BYTE);
  if(status == MM_STATUS_OK)
   break;


  //[1]如果地址是4096Bytes碎片地址，则释放内存
  status = mm_management_node_free(pmm_fix_4096_head, mm_ptr, C_MM_4096BYTE);
  if(status == MM_STATUS_OK)
   break;
 }while(0);


 if(status == MM_STATUS_OK)
 {
  //do nothing,在mm_management_node_free函数中已经将pmm_fix_node->iflag设为MM_STATUS_FREE
 }
 else
 {
  free(mm_ptr);
 }
}


//获取MM_SDRAM_STRUCT里的空闲节点
static P_MM_Node_STRUCT mm_management_getnode(P_MM_SDRAM_STRUCT pmm_fix_head)
{
 P_MM_SDRAM_STRUCT pmm_fix_head_tmp = pmm_fix_head;
 P_MM_Node_STRUCT  pmm_fix_node = pmm_fix_head_tmp->next;
 unsigned int count = pmm_fix_head_tmp->count;
 unsigned int i;


 for(i=0; iiflag == MM_STATUS_FREE)
   break;
  pmm_fix_node = pmm_fix_node->next;
 }


 if(i < count)
  return pmm_fix_node;
 else
  return NULL;
}


//比较MM_SDRAM_STRUCT的所有节点，如果地址一致，则释放地址
static unsigned int mm_management_node_free(P_MM_SDRAM_STRUCT pmm_fix_head, unsigned int *mm_ptr, unsigned int size)
{
 P_MM_SDRAM_STRUCT pmm_fix_head_tmp = pmm_fix_head;
 P_MM_Node_STRUCT  pmm_fix_node = pmm_fix_head_tmp->next;
 unsigned int count = pmm_fix_head_tmp->count;
 unsigned int i;


 for(i=0; imm_node == mm_ptr)
  {
   if(pmm_fix_node->iflag == MM_STATUS_FREE)
   {
    printf("mm management have been free
");
   }
   else
   {
    pmm_fix_node->iflag = MM_STATUS_FREE;
    memset((void *)mm_ptr, 0x00, size);    //释放内存后，把碎片内存清0
   }
   
   return MM_STATUS_OK;
  }
  pmm_fix_node = pmm_fix_node->next;
 }


 return MM_STATUS_FAIL;
}

  这份代码我写得还是比较简单，注释些也写得清楚，明白它的原理，应该很容易就看懂。

 

说一下这个机制的优缺点：

优点：

小内存申请的时候，先去提前申请好的内存中获取，这样可以很好地解决内存碎片问题。

缺点以及优化：

1.碎片管理机制可申请的碎片数量是有限的，当数量被申请完之后，还是得重新用malloc申请；但是这可以通过我定义的 C_MM_16BYTE_NUM 和 C_MM_16BYTE 这些宏定义修改碎片数量，根据项目需要修改数量，也是能很好的优化此问题；

2.比如我要申请4个Bytes，但此时，16，64，256，512，1024这几个链表已经用完了，那此时它会用4096这个链表去给4Bytes使用，当然，这同样可以修改C_MM_16BYTE_NUM 和 C_MM_16BYTE 这些宏定义优化这个问题。

　　审核编辑：汤梓红