自定义内存分配器

时间：2011-07-03 17:52:42

标签： c arrays memory pointers memory-management

我写了一个自定义内存分配器。它有2个限制，我想删除所以它的工作方式类似malloc / free。

1。）mem_free调用需要为其输入参数强制转换为unsigned char *。我希望它采用任何类型的指针。怎么办呢？

2.。）我写的内存分配器为缓冲区的前面分配了一块内存，并且还写入了它的大小。 free函数删除缓冲区中最后一块已分配的内存。所以malloc / free调用的顺序很重要或者它不起作用。如何删除此限制？

希望能够做到这一点：

char* ptr1 = mem_alloc(10);
char* ptr2 = mem_alloc(4);

mem_free(ptr1);
mem_free(ptr2);

现在必须这样做：

char* ptr1 = mem_alloc(10);
char* ptr2 = mem_alloc(4);

mem_free(ptr2);
mem_free(ptr1);

代码：

unsigned char* mem_alloc(unsigned int size) {
    unsigned int s;
    if( (size + MEM_HEADER_SIZE)  > (MEM_MAX_SIZE - mem_current_size_bytes) ) {
        return NULL;
    }
    if(is_big_endian() == 0) {
        s = (mem_buff[3] << 24) + (mem_buff[2] << 16) + (mem_buff[1] << 8) + mem_buff[0];
    } else {
        s = (mem_buff[0] << 24) + (mem_buff[1] << 16) + (mem_buff[2] << 8) + mem_buff[3];
    }
    memcpy(mem_buff + mem_current_size_bytes, &size, sizeof(unsigned int));
    unsigned char* result = mem_buff + (mem_current_size_bytes + MEM_HEADER_SIZE);
    mem_current_size_bytes += MEM_HEADER_SIZE + size;
    return result;
}

void mem_free(unsigned char* ptr) {
    unsigned int i,s;
    for(i=0; i<mem_current_size_bytes; i++) {
        if( (char*)ptr == (char*)(mem_buff + i) ) {
            if(is_big_endian() == 0) {
                s = (*(ptr - 1) << 24) + (*(ptr - 2) << 16) + (*(ptr - 3) << 8) + *(ptr - 4);
            } else {
                s = (*(ptr - 4) << 24) + (*(ptr - 3) << 16) + (*(ptr - 2) << 8) + *(ptr - 1);
            }
            mem_current_size_bytes-=s;
            mem_current_size_bytes-=MEM_HEADER_SIZE;
            break;
        }
    }
}

4 个答案:

答案 0 :(得分：5)

1）改为使用void * 2）将地址映射存储到已分配的块和未分配的块的单独映射。然后，您可以在分配的映射中查找正在释放的块，将其删除，然后将块添加到未分配的映射（确保将其与任何一侧的任何空闲块合并）。当然，这可能而且确实会导致内存碎片，但实际上这是不可避免的。

答案 1 :(得分：3)

只需在mem_free的参数中使用void *指针而不是char *。

为了使内存分配器能够在任何内存位置工作，你需要增加更多的复杂性...我建议研究如何管理内存堆，你会发现一些基本的方案来试用。

答案 2 :(得分：3)

你写道，你正在寻找想法，所以我附上了我在大学完成的一个项目，我们应该实现malloc（）和free（）...... 您可以看到我是如何做到这一点的，如果您愿意，可能会获得灵感或使用它。它肯定不是最快的实现，但相当容易， 应该没有错误; - ）

（请注意，如果来自同一课程的人偶然遇到过这种情况 - 请不要使用它而是自己制作 - 因此许可证; - ）

/*
 * The 1st project for Data Structures and Algorithms course of 2010
 *
 * The Faculty of Informatics and Information Technologies at
 * The Slovak University of Technology, Bratislava, Slovakia
 *
 *
 * Own implementation of stdlib's malloc() and free() functions
 *
 * Author: mnicky
 *
 *
 * License: modified MIT License - see the section b) below
 *
 * Copyright (C) 2010 by mnicky
 *
 * Permission is hereby granted, free of charge, to any person obtaining a copy
 * of this software and associated documentation files (the "Software"), to deal
 * in the Software without restriction, including without limitation the rights
 * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
 * copies of the Software, and to permit persons to whom the Software is
 * furnished to do so, subject to the following conditions:
 *
 * a) The above copyright notice and this permission notice - including the
 *    section b) - shall be included in all copies or substantial portions
 *    of the Software.
 *
 * b) the Software WILL NOT BE USED IN ANY WORK DIRECTLY OR INDIRECTLY
 *    CONNECTED WITH The Faculty of Informatics and Information Technologies at
 *    The Slovak University of Technology, Bratislava, Slovakia
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
 * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
 * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
 * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
 * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
 * THE SOFTWARE.
 *
 */

#include <stdio.h>

typedef unsigned int MEMTYPE;

MEMTYPE *mem;
MEMTYPE memSize;
MEMTYPE avail;  //1st index of the 1st free range

//return size of block
MEMTYPE blockSize(MEMTYPE x) {
  return mem[x];
}

//return next free block
MEMTYPE next(MEMTYPE x) {
  return mem[x + mem[x]];
}

//return index of pointer to next free block
MEMTYPE linkToNext(MEMTYPE x) {
  return x + mem[x];
}

//initialize memory
void my_init(void *ptr, unsigned size) {

  mem = (MEMTYPE *) ptr;
  memSize = size / sizeof(MEMTYPE);
  mem[0] = memSize - 1;
  mem[memSize - 1] = memSize;
  avail = 0;
}

//allocate memory
void *my_alloc(unsigned size) {

  if (size == 0) {  //return NULL pointer after attempt to allocate 0-length memory
    return NULL;
  }

  MEMTYPE num = size / sizeof(MEMTYPE);
  if (size % sizeof(MEMTYPE) > 0) num++;
  MEMTYPE cur, prev;  //pointer to (actually index of) current block, previous block
  MEMTYPE isFirstFreeBeingAllocated = 1;  //whether the first free block is being allocated

  prev = cur = avail;

  //testing, whether we have enough free space for allocation
  test:

  if (avail == memSize) {  //if we are on the end of the memory
    return NULL;
  }

  if (blockSize(cur) < num) {  //if the size of free block is lower than requested
    isFirstFreeBeingAllocated = 0;
    prev = cur;

    if (next(cur) == memSize) {  //if not enough memory
      return NULL;
    }
    else
      cur = next(cur);
    goto test;
  }

  if (blockSize(cur) == num) {  //if the size of free block is equal to requested

    if (isFirstFreeBeingAllocated)
      avail = next(cur);
    else
      mem[linkToNext(prev)] = next(cur);
  }

  else {  //if the size of free block is greater than requested

    if (isFirstFreeBeingAllocated) {
      if ((blockSize(cur) - num) == 1)  //if there is only 1 free item left from this (previously) free block
        avail = next(cur);
      else
        avail = cur + num + 1;
    }
    else {
      if ((blockSize(cur) - num) == 1)  //if there is only 1 free item left from this (previously) free block
        mem[linkToNext(prev)] = next(cur);
      else
        mem[linkToNext(prev)] = cur + num + 1;
    }

    if ((blockSize(cur) - num) == 1)  //if there is only 1 free item left from this (previously) free block
      mem[cur] = num + 1;
    else {
      mem[cur + num + 1] = blockSize(cur) - num - 1;
      mem[cur] = num;
    }
  }

  return (void *) &(mem[cur+1]);
}

//free memory
void my_free(void *ptr) {

  MEMTYPE toFree;  //pointer to block (to free)
  MEMTYPE cur, prev;

  toFree = ((MEMTYPE *)ptr - (mem + 1));


  if (toFree < avail) {  //if block, that is being freed is before the first free block

    if (((linkToNext(toFree) + 1) == avail) && avail < memSize)  //if next free block is immediately after block that is being freed
      mem[toFree] += (mem[avail] + 1);  //defragmentation of free space
    else
      mem[linkToNext(toFree)] = avail;

    avail = toFree;
  }

  else {  //if block, that is being freed isn't before the first free block

    prev = cur = avail;

    while (cur < toFree) {
      prev = cur;
      cur = next(cur);
    }

    if ((linkToNext(prev) + 1) == toFree) { //if previous free block is immediately before block that is being freed

      mem[prev] += (mem[toFree] + 1);  //defragmentation of free space

      if (((linkToNext(toFree) + 1) == cur) && cur < memSize)  //if next free block is immediately after block that is being freed
        mem[prev] += (mem[cur] + 1);  //defragmentation of free space
      else
        mem[linkToNext(toFree)] = cur;
    }
    else {
      mem[linkToNext(prev)] = toFree;
      mem[linkToNext(toFree)] = cur;
    }

  }
}

尽可能使用少量空间进行元信息传输，因此分配的空间由分配范围的第一个节点中的数字标记，表示后面分配的节点数。

自由范围内的可用空间量由一个数字标记，表示后续（包括该节点）的空闲节点数，自由范围的最后一个节点包含下一个自由范围的第一个索引的数量 - 这样的（红色空间被分配，白色是免费的）：

enter image description here

它可以像这样使用：

char region[30000000];  //space for memory allocation
my_init(region, 30000000);  //memory initialization

var = (TYPE *) my_alloc(sizeof(TYPE));  //memory allocation
my_free((void *) var);  //freeing the memory

答案 3 :(得分：2)

  1. 你必须重写整个代码。内存分配器通常使用链表来存储已使用和未使用的内存块，您可以将此链接放在大小之前的块的标头中。我建议你搜索一些关于内存分配器如何工作的文章。编写性能良好的内存分配器是 hard 任务。
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