一個簡化版本的記憶體池實現
阿新 • • 發佈：2019-02-07
最近寫的一個程式中需要頻繁的申請和釋放零碎的記憶體塊，這些記憶體塊的大小卻只有簡單的幾種。如果直接使用系統呼叫malloc/free、new/delete進行記憶體分配和釋放，則效率很低。程式執行時間長了會產生大量的記憶體碎片。想起uC/OS-II 裡面提供了個記憶體分配單元，正好滿足我的需要。就把裡面相關的程式碼扒了出來。寫成了一個記憶體池的類。
這個記憶體池的功能非常的簡單，初始化時分配一大塊記憶體，然後將各個小記憶體塊串成一個單項鍊表。每次分配記憶體塊時從連結串列的頭上去取一個記憶體塊。回收記憶體塊時也是將記憶體塊插到連結串列的開頭處。
這個類的結構如下：
#ifndef MEMPOOL_H #define MEMPOOL_H #define MEM_NO_ERR 0 #define MEM_INVALID_ADDR 1 #define MEM_INVALID_BLKS 2 #define MEM_INVALID_SIZE 3 #define MEM_INVALID_PART 4 #define MEM_INVALID_PBLK 5 #define MEM_FULL 6 class MemPool { public: MemPool(); ~MemPool(); int create (int nblocks, unsigned int blocksize); void* get( void ); int release ( void *pblk ); int blocks( void ) const {return m_memNBlks;}; int frees( void ) const {return m_memNFree;}; private: char *m_memAddr; /* Pointer to beginning of memory partition */ char *m_memFreeList; /* Pointer to list of free memory blocks */ int m_memBlkSize; /* Size (in bytes) of each block of memory */ int m_memNBlks; /* Total number of blocks in this partition */ int m_memNFree; /* Number of memory blocks remaining in this */ }; #endif
create 函式初始化記憶體池。主要的工作就是分配記憶體，然後將記憶體塊串起來形成一個連結串列。因為要用指標形成連結串列，因此要求記憶體塊的大小至少要能容納一個指標。
get 函式獲得一個記憶體塊，如果沒有剩餘的記憶體塊了，就返回 null。
release 函式回收記憶體塊。
blocks 函式返回記憶體池總共有多少記憶體塊。
frees 函式返回記憶體池還剩多少剩餘的記憶體塊。
程式碼實現如下：
#include <stddef.h> #include "MemPool.h" MemPool::MemPool() { m_memAddr = NULL; m_memFreeList = NULL; m_memBlkSize = 0; m_memNBlks = 0; m_memNFree = 0; } MemPool::~MemPool() { if(m_memAddr != NULL) { delete [] m_memAddr; } } int MemPool::create ( int nblks, unsigned int blksize ) { if( m_memAddr != NULL ) { delete [] m_memAddr; } m_memAddr = new char[nblks * blksize]; if ( m_memAddr == NULL ) { return MEM_INVALID_ADDR; } if ( nblks < 2 ) { /* Must have at least 2 blocks per partition */ return MEM_INVALID_BLKS; } if ( blksize < sizeof(void *) ) { /* Must contain space for at least a pointer */ return MEM_INVALID_SIZE; } void ** p = (void **)m_memAddr; char *pblk = m_memAddr + blksize; for (int i = 0; i < (nblks - 1); i++) { *p = (void *) pblk; p = (void **) pblk; pblk = pblk + blksize; } *p = (void *)0; m_memFreeList = m_memAddr; m_memNBlks = nblks; m_memNFree = nblks; return MEM_NO_ERR; } void * MemPool::get( void ) { void *pblk; if (m_memNFree > 0) { /* See if there are any free memory blocks */ pblk = m_memFreeList; /* Yes, point to next free memory block */ m_memFreeList = (char *) *(void **)pblk; /* Adjust pointer to new free list */ m_memNFree--;/* One less memory block in this partition */ return (pblk); /* Return memory block to caller */ } return ((void *)0); } int MemPool::release ( void *pblk ) { if (pblk == (void *)0) { /* Must release a valid block */ return (MEM_INVALID_PBLK); } if (m_memNFree >= m_memNBlks) { /* Make sure all blocks not already returned */ return (MEM_FULL); } /* Insert released block into free block list */ *(void **)pblk = m_memFreeList; m_memFreeList = (char *) pblk; m_memNFree++; /* One more memory block in this partition */ return (MEM_NO_ERR); /* Notify caller that memory block was released */ }
下面是個簡答的測試程式碼：
#include <iostream> #include "MemPool.h" using namespace std; typedef struct { unsigned char *ImageData; int SizeX; int SizeY; int ImageID; double Timestamp; double TransferTime; unsigned int PacketCount; }IMAGE_INFO; int main() { IMAGE_INFO * p[15]; MemPool mem; mem.create(15, sizeof(IMAGE_INFO)); for(int i = 0; i < 10; i++) { p[i] = (IMAGE_INFO *)mem.get(); cout << "p[" << i << "] addr = " << p[i] << endl; p[i]->SizeX = i; p[i]->SizeY = i; } for(int i = 1; i < 10; i++) { cout << "p[" << i << "]->SizeX = " << p[i]->SizeX << endl; mem.release(p[i]); } cout << "mem.blocks()" << mem.blocks() << endl; cout << "mem.frees()" << mem.frees() << endl; return 0; }
這個測試程式的執行結果如下：
p[0] addr = 0xac1358
p[1] addr = 0xac1380
p[2] addr = 0xac13a8
p[3] addr = 0xac13d0
p[4] addr = 0xac13f8
p[5] addr = 0xac1420
p[6] addr = 0xac1448
p[7] addr = 0xac1470
p[8] addr = 0xac1498
p[9] addr = 0xac14c0
p[1]->SizeX = 1
p[2]->SizeX = 2
p[3]->SizeX = 3
p[4]->SizeX = 4
p[5]->SizeX = 5
p[6]->SizeX = 6
p[7]->SizeX = 7
p[8]->SizeX = 8
p[9]->SizeX = 9
mem.blocks()15
mem.frees()14最後多說一句，如果程式中多個執行緒要訪問同一個記憶體池，那個需要給 get 和 release 函式加鎖。
另外，這個程式碼其實可以用C++的類模版來實現。下面是用了類模版技術的一個實現：
#ifndef MEMPOOL2_H_INCLUDED
#define MEMPOOL2_H_INCLUDED
#define MEM_NO_ERR 0
#define MEM_INVALID_ADDR 1
#define MEM_INVALID_BLKS 2
#define MEM_INVALID_SIZE 3
#define MEM_INVALID_PART 4
#define MEM_INVALID_PBLK 5
#define MEM_FULL 6
template < typename T >
class MemPool2
{
public:
MemPool2();
~MemPool2();
int create (int nblocks);
T* get( void );
int release ( T *pblk );
int blocks( void ) const {return m_NBlocks;};
int frees( void ) const {return m_NFree;};
private:
T *m_addr; /* Pointer to beginning of memory partition */
void *m_freeList; /* Pointer to list of free memory blocks */
int m_blockSize; /* Size (in bytes) of each block of memory */
int m_NBlocks; /* Total number of blocks in this partition */
int m_NFree; /* Number of memory blocks remaining in this */
};
template < typename T >
MemPool2<T>::MemPool2()
{
m_addr = NULL;
m_freeList = NULL;
m_blockSize = sizeof(T);
m_NBlocks = 0;
m_NFree = 0;
}
template < typename T >
MemPool2<T>::~MemPool2()
{
if(m_addr != NULL)
{
delete [] m_addr;
}
}
template < typename T >
int MemPool2<T>::create ( int nblocks )
{
if( m_addr != NULL )
{
delete [] m_addr;
}
m_addr = new T[nblocks];
if ( m_addr == NULL )
{
return MEM_INVALID_ADDR;
}
if ( nblocks < 2 )
{
/* Must have at least 2 blocks per partition */
return MEM_INVALID_BLKS;
}
if ( sizeof(T) < sizeof(void *) )
{
/* Must contain space for at least a pointer */
return MEM_INVALID_SIZE;
}
void ** p = (void **)m_addr;
T *pblock = m_addr;
for (int i = 0; i < (nblocks - 1); i++)
{
pblock ++;
*p = (void *) pblock;
p = (void **) pblock;
}
*p = (void *)0;
m_freeList = m_addr;
m_NBlocks = nblocks;
m_NFree = nblocks;
return MEM_NO_ERR;
}
template < typename T >
T * MemPool2<T>::get( void )
{
T *pblk;
if (m_NFree > 0)
{
/* See if there are any free memory blocks */
pblk = (T *) m_freeList; /* Yes, point to next free memory block */
m_freeList = (void *) *(void **)pblk; /* Adjust pointer to new free list */
m_NFree--;/* One less memory block in this partition */
return (pblk); /* Return memory block to caller */
}
return ((T *)0);
}
template < typename T >
int MemPool2<T>::release ( T *pblock )
{
if (pblock == NULL)
{
/* Must release a valid block */
return (MEM_INVALID_PBLK);
}
if (m_NFree >= m_NBlocks)
{
/* Make sure all blocks not already returned */
return (MEM_FULL);
}
/* Insert released block into free block list */
*(void **)pblock = m_freeList;
m_freeList = (void *) pblock;
m_NFree++; /* One more memory block in this partition */
return (MEM_NO_ERR); /* Notify caller that memory block was released */
}
#endif // MEMPOOL2_H_INCLUDED
測試程式碼如下：
#include <iostream>
#include "MemPool2.h"
using namespace std;
typedef struct
{
unsigned char *ImageData;
int SizeX;
int SizeY;
int ImageID;
double Timestamp;
double TransferTime;
unsigned int PacketCount;
}IMAGE_INFO;
int main()
{
IMAGE_INFO * p[15] = {0};
MemPool2<IMAGE_INFO> mem;
mem.create(5);
for(int i = 0; i < 7; i++)
{
p[i] = mem.get();
if(p[i] != NULL)
{
cout << "p[" << i << "] addr = " << p[i] << endl;
p[i]->SizeX = i;
p[i]->SizeY = i;
}
}
for(int i = 0; i < 9; i++)
{
if(p[i] != NULL)
{
cout << "p[" << i << "]->SizeX = " << p[i]->SizeX << endl;
}
mem.release(p[i]);
}
cout << "mem.blocks()" << mem.blocks() << endl;
cout << "mem.frees()" << mem.frees() << endl;
return 0;
}