C语言编写可以实现malloc() & free()功能的函数（空间/时间复杂度低）

#include <stdio.h>

typedef struct _BLOCK   //定义一个结构体，用于记录所有可用区块的大小及位置
{
    unsigned int size;
    struct _BLOCK xdata* pLink;

}BLOCK,xdata* PBLOCK;

PBLOCK xdata pHead = NULL;

// 
// Initialize 
//    Init the Block  
// 
// Formal Parameters: 
//    memory = the address of memory
//    len = the size of the memory   
// 
// Global Input Parameters: 
//    None
// 
// Global Output Parameters: 
//    None 
// 
// Return Value: 
//    None 
//
void 
MemInitializePool(
    void* memory,
    unsigned len
    )
{
    pHead = (PBLOCK)memory;   //头结点指向自定义用于分配内存的空间
    pHead->pLink = NULL;
    pHead->size = len;
}

// 
// MyMalloc 
//    malloc the space  
// 
// Formal Parameters: 
//    needSize = size of the Block we need    
// 
// Global Input Parameters: 
//    None
// 
// Global Output Parameters: 
//    None 
// 
// Return Value: 
//    result = the location of the Block 
//
void* 
MemAlloc(
    unsigned int needSize
    )
{
    PBLOCK pPrior,pNext,pNewNode;
    void* result;
    pNext  = pHead;
    pPrior = pHead;
    do
    {
        if ( pNext->size >= needSize )     //寻找到合适大小的空白区块
        {
            pNext->size    = pNext->size - needSize - sizeof(BLOCK);  //分配空间出去
            pNewNode       = (PBLOCK)((void*)pNext + pNext->size + sizeof(BLOCK));//得到存储信息的空间的地址
            pNewNode->size = needSize; //可使用的空间大小
                        
            if(pNext->size < sizeof(BLOCK))
            {
                pPrior->pLink = pNext->pLink;
            }

            result = (void*)(++pNewNode);//返回分配出去的空间地址
            return result;
        }

        pPrior = pNext;
        pNext  = pNext->pLink;
    
    }while ( pNext != NULL );
    if(pNext == NULL)                //over
    {
        return NULL;
    }    
}


// 
// merge 
//    merge the Block we needn't use  
// 
// Formal Parameters: 
//    pPrior = the node of the Link
//    pNext = the pLink node of the Link    
// 
// Global Input Parameters: 
//    None
// 
// Global Output Parameters: 
//    None 
// 
// Return Value: 
//    None 
//
void 
Merge(
    PBLOCK pPrior,
    PBLOCK pNext
    )
{
    if ( pNext == (PBLOCK)((void*)pPrior + pPrior->size + sizeof(PBLOCK)) ) //如果两个空白区块连续，合并空间
    {
        pPrior->size += (pNext->size) + sizeof(BLOCK);
        pPrior->pLink = pNext->pLink;
    }
}

// 
// MemFree 
//    free the Block we used  
// 
// Formal Parameters: 
//    pBlock = the location of the Block we need to free  
// 
// Global Input Parameters: 
//    None
// 
// Global Output Parameters: 
//    None 
// 
// Return Value: 
//    None 
//
void 
MemFree(
    PBLOCK pBlock
    )
{
    PBLOCK pFree  = (--pBlock);
    PBLOCK pNext  = pHead;
    PBLOCK pPrior = pHead;

    do
    {
        pPrior = pNext;
        pNext  = pNext->pLink;

        if( pPrior <= pFree && pNext >= pFree)  //找到所需要释放的区块的位置
        {
            pFree->pLink = pNext;
            pPrior->pLink = pFree;
            break;
        }

    }while ( pNext != NULL );
    //合并空白区块
    Merge(
        pPrior,
        pFree
        );
    Merge(
        pFree,
        pNext
        );
}