Question

programming language: C
platform: ARM
Compiler: ADS 1.2

I need to keep track of simple melloc/free calls in my project. I just need to get very basic idea of how much heap memory is required when the program has allocated all its resources. Therefore, I have provided a wrapper for the malloc/free calls. In these wrappers I need to increment a current memory count when malloc is called and decrement it when free is called. The malloc case is straight forward as I have the size to allocate from the caller. I am wondering how to deal with the free case as I need to store the pointer/size mapping somewhere. This being C, I do not have a standard map to implement this easily.

I am trying to avoid linking in any libraries so would prefer *.c/h implementation.

So I am wondering if there already is a simple implementation one may lead me to. If not, this is motivation to go ahead and implement one.

EDIT: Purely for debugging and this code is not shipped with the product.

EDIT: Initial implementation based on answer from Makis. I would appreciate feedback on this.

EDIT: Reworked implementation

#include <stdlib.h>
#include <stdio.h>
#include <assert.h>
#include <string.h>
#include <limits.h>

static size_t gnCurrentMemory = 0;
static size_t gnPeakMemory    = 0;

void *MemAlloc (size_t nSize)
{
  void *pMem = malloc(sizeof(size_t) + nSize);

  if (pMem)
  {
    size_t *pSize = (size_t *)pMem;

    memcpy(pSize, &nSize, sizeof(nSize));

    gnCurrentMemory += nSize;

    if (gnCurrentMemory > gnPeakMemory)
    {
      gnPeakMemory = gnCurrentMemory;
    }

    printf("PMemAlloc (%#X) - Size (%d), Current (%d), Peak (%d)\n",
           pSize + 1, nSize, gnCurrentMemory, gnPeakMemory);

    return(pSize + 1);
  }

  return NULL;
}

void  MemFree (void *pMem)
{
  if(pMem)
  {
    size_t *pSize = (size_t *)pMem;

    // Get the size
    --pSize;

    assert(gnCurrentMemory >= *pSize);

    printf("PMemFree (%#X) - Size (%d), Current (%d), Peak (%d)\n",
           pMem,  *pSize, gnCurrentMemory, gnPeakMemory);

    gnCurrentMemory -= *pSize;

    free(pSize);
  }
}

#define BUFFERSIZE (1024*1024)

typedef struct
{
  bool flag;
  int buffer[BUFFERSIZE];
  bool bools[BUFFERSIZE];
} sample_buffer;

typedef struct
{
  unsigned int whichbuffer;
  char ch;
} buffer_info;


int main(void)
{
  unsigned int i;
  buffer_info *bufferinfo;

  sample_buffer  *mybuffer;

  char *pCh;

  printf("Tesint MemAlloc - MemFree\n");

  mybuffer = (sample_buffer *) MemAlloc(sizeof(sample_buffer));

  if (mybuffer == NULL)
  {
    printf("ERROR ALLOCATING mybuffer\n");

    return EXIT_FAILURE;
  }

  bufferinfo = (buffer_info *) MemAlloc(sizeof(buffer_info));

  if (bufferinfo == NULL)
  {
    printf("ERROR ALLOCATING bufferinfo\n");

    MemFree(mybuffer);

    return EXIT_FAILURE;
  }

  pCh = (char *)MemAlloc(sizeof(char));

  printf("finished malloc\n");

  // fill allocated memory with integers and read back some values
  for(i = 0; i < BUFFERSIZE; ++i)
  {
    mybuffer->buffer[i] = i;
    mybuffer->bools[i] = true;
    bufferinfo->whichbuffer = (unsigned int)(i/100);
  }


  MemFree(bufferinfo);
  MemFree(mybuffer);

  if(pCh)
  {
    MemFree(pCh);
  }

  return EXIT_SUCCESS;
}

Answer 1

You could allocate a few extra bytes in your wrapper and put either an id (if you want to be able to couple malloc() and free()) or just the size there. Just malloc() that much more memory, store the information at the beginning of your memory block and and move the pointer you return that many bytes forward.

This can, btw, also easily be used for fence pointers/finger-prints and such.