自定义malloc()

时间:2016-03-14 01:45:31

标签: c memory dynamic allocation

我正在尝试在C中实现自己的malloc()函数,但我遇到了这个问题。前两个分配的地址是正确的,但在此之后,它不会显示其他地址。我的意思是,它陷入无限循环。但是,如果我删除条件&& ptr->size > BLK_SIZE,它似乎有效。所以,问题是,为什么这个条件破坏了代码?如何通过除去条件之外的其他方式解决这个问题?

这是我的代码......

/* Author : Singh*/

typedef struct block_header {
    unsigned int size : 29,
                 zero : 2,
                alloc : 1;
} block_header;

//macros
#define HEADER_BLK_SIZE sizeof(block_header) // 4 bytes header
#define ALIGNED_PAYLOAD_SIZE (((((size)-1)>>2)<<2)+4) //to align the payload
#define BLK_SIZE HEADER_BLK_SIZE + ALIGNED_PAYLOAD_SIZE //total size of a blk
#define HEAP_EXTEND_SIZE ((BLK_SIZE)*1024) //the total heap size

static void *base_heap = NULL; //base of the heap, starting point
static void *end_heap = NULL;
static block_header *freeblk = NULL;

void *mymalloc(size_t size) {
    size_t remainder_heap = (HEAP_EXTEND_SIZE) - (BLK_SIZE);

    // first time init the heap and allocate the first block
    if (!base_heap) {
        base_heap = sbrk(0);
        end_heap = sbrk(HEAP_EXTEND_SIZE);
        if (base_heap == (void*)-1 || end_heap == (void*)-1)
            return NULL;

        block_header *blk = (block_header*)base_heap;
        blk->size = BLK_SIZE;
        blk->zero = 2;
        blk->alloc = 1;

        freeblk = ((void*)(base_heap)) + BLK_SIZE;
        freeblk->size = remainder_heap;
        freeblk->zero = 2;
        freeblk->alloc = 0;

        return ((void*)blk) + HEADER_BLK_SIZE;
    } else
    if (size >= HEAP_EXTEND_SIZE) {
        return NULL;
    } else {
    //second time and the others
        block_header *ptr = (block_header*)base_heap;
        size_t i;
        i = 0;
        while (i < (HEAP_EXTEND_SIZE)) { //travel the heap
            if ((ptr->alloc) ==1 ) { //if it's allocate we go to the nxt block
                ptr = ((void*)ptr) + ((size_t)(ptr->size));
                i += ((size_t)(ptr->size));
            } else
            if ((ptr->alloc) == 0 && ptr->size > BLK_SIZE) { /*if it's free and
                                                              big enough */
                ptr->size = BLK_SIZE;
                ptr->zero = 2;
                ptr->alloc = 1;
                return ((void*)ptr) + (HEADER_BLK_SIZE);
            } else { //not big enough so we go to the next block
                ptr = ((void*)ptr) + ((size_t)(ptr->size));
                i += ((size_t)(ptr->size));
            }
        }
        return NULL; //if it does not wok
    }
}

//for testing my code
void main() {
    int *i =(int*)mymalloc(12);
    printf("pointeur i : %p\n", i);

    int *ii = (int*)mymalloc(16);
    printf("pointeur ii : %p\n", ii);

    int *iii = (int*)mymalloc(20);
    printf("pointeur iii : %p\n", iii);

    int *iiii = (int*)mymalloc(24);
    printf("pointeur iiii : %p\n", iiii);
}

3 个答案:

答案 0 :(得分:1)

您的代码存在一个主要问题:

#define ALIGNED_PAYLOAD_SIZE (((((size)-1)>>2)<<2)+4) //to align the payload
#define BLK_SIZE HEADER_BLK_SIZE + ALIGNED_PAYLOAD_SIZE //total size of a blk
#define HEAP_EXTEND_SIZE ((BLK_SIZE)*1024) //the total heap size

所有这些宏都引用了一些当前的size变量。 sizemymalloc的参数名称,使所有这些标识符不一致。这可能不是你想要的......

例如,看起来无辜的测试:

if(size >= HEAP_EXTEND_SIZE)

实际上扩展为

if(size >= ((HEADER_BLK_SIZE + (((((size)-1)>>2)<<2)+4))*1024))

除非发生无符号算术溢出,否则这是假的,并且巧合地计算为真。

您必须先清理代码,然后才能进行有效调试。

答案 1 :(得分:1)

  

在我看来,根本原因是你在分配一个块后忘记处理freeblock点。你需要处理freeblock点的原因是你需要freeblock来做出判断。   以下是我的修改:

    else if((ptr->alloc)==0 && ptr->size > BLK_SIZE){ /*if it's free and
                                                        big enough */
        ptr->size = BLK_SIZE;
        ptr->zero = 2;
        ptr->alloc = 1;
        freeblk = ((void *)ptr) + (size_t)(ptr->size);
        freeblk->size = freeblk->size - ptr->size;
        freeblk->zero = 2;
        freeblk->alloc = 0;
        return ((void*)ptr) + (size_t)(ptr->size);
    }
  

测试结束后,我发现它有效。   enter code here [zzhen201 @〜] $ ./test   pointeur i:0x2097004   pointeur ii:0x2097024   pointeur iii:0x209703c   pointeur iiii:0x2097058

但是,我没有添加更多测试来测试是否还有其他错误。 所以也许你需要做更多的工作来测试它。

答案 2 :(得分:0)

如果我改变这一部分:

  else{
      block_header* ptr = (block_header*) base_heap;
      size_t i;
      i = 0;
      while(i<(HEAP_EXTEND_SIZE)){ //travel the heap
        if((ptr->alloc)==1){ //if it's allocate we go to the nxt block
          ptr = ((void*)ptr) + ((size_t)(ptr->size));
          i += ((size_t)(ptr->size));
        }
        else if((ptr->alloc)==0 && ptr->size > BLK_SIZE){ /*if it's free and
                                                            big enough */
            ptr->size = BLK_SIZE;
            ptr->zero = 2;
            ptr->alloc = 1;
            return ((void*)ptr) + (HEADER_BLK_SIZE);
        }
        else{ //not big enough so we go to the next block
          ptr = ((void*)ptr) + ((size_t)(ptr->size));
          i += ((size_t)(ptr->size));
        }
      }
      return NULL; //if it does not wok
    }

这一部分(如果空闲块足够大,它就不知道了)它的工作原理是&#34;我想..

else{
      block_header* ptr = (block_header*) base_heap;
      size_t i;
      i = 0;
      while(i<(HEAP_EXTEND_SIZE)){
        if((ptr->alloc)==1){
          ptr = ((void*)ptr) + ((size_t)(ptr->size));
          i += ((size_t)(ptr->size));
        }
        else{
          ptr->size = BLK_SIZE;
          ptr->zero = 2;
          ptr->alloc = 1;
          return ((void*)ptr) + (HEADER_BLK_SIZE);
        }
      }
      return NULL;
    }
    return NULL;
  }