Question

#pragma pack(push, 1)
typedef struct 
{
      /*...*/
  unsigned int      dataoffset;         //No of bytes before actual pixel data
}HEADER;

typedef struct 
{
   /*...*/
   unsigned int     width;
   unsigned int     height;
   unsigned short   bits_per_pixel;         //code is written for 24 bits only and No other format is supported..
    /*...*/
}INFO_HEADER;

typedef struct 
{
   unsigned char    b;
   unsigned char    g;
   unsigned char    r;        
}COLORMAP;

#pragma pack(pop)


int main()
{
      // Var decl.
  INFO_HEADER       *pHeader = NULL;
  FILE              *pImage;
  COLORMAP          **ppColors;
  /*...*/

/* File opened in read, binary mode, memory allocated for pHeader*/     

fread (pHeader, sizeof(INFO_HEADER), 1, pImage);  

/*Next block is actually problematic.. Posting 'as is' from my code*/
      ppColors = (COLORMAP**)malloc((pHeader -> height ) * sizeof(COLORMAP));
  for(i = 0 ; i < pHeader -> height ; i++)
     ppColors[i] = (COLORMAP*)malloc(pHeader -> width * sizeof(COLORMAP));
fseek(pImage, pHeader -> fileheader.dataoffset, SEEK_SET);
    for (i = 0 ; i < pHeader -> width ; i++) 
  {
      for (j = 0 ; j < pHeader -> height ; j++) 
      {
          fread(&b, sizeof(unsigned char), 1, pImage);
          fread(&g, sizeof(unsigned char), 1, pImage);
          fread(&r, sizeof(unsigned char), 1, pImage);

          ppColors[i][j].b = b;
          ppColors[i][j].g = g;
          ppColors[i][j].r = r; 

          printf("width = %d height = %d %d:\t", i, j,  cnt);
          printf("%d ", (int)ppColors[i][j].b);
          printf("%d ", (int)ppColors[i][j].g);
          printf("%d\n", (int)ppColors[i][j].r);
          cnt++;  
      }
  }

  /*And at last free()ing..*/
  for(i = 0 ; i < pHeader -> height ; i++)        free(ppColors[i]);
   free(ppColors);
  cleanup();
  return(0)
}

Possible Duplicate: http://stackoverflow.com/questions/1568042/optimal-way-to-free-a-malloced-2d-array-in-c

Though above link could not solve my problems.

1. I run out of memory. I have malloc()ed for height, then for each of the height, width is again malloc()ed. I am trying to work on width X height domain only. It seems that problem is with height. If you change

ppColors = (COLORMAP**)malloc((pHeader -> height ) * sizeof(COLORMAP)); to
ppColors = (COLORMAP**)malloc((pHeader -> height + 6 ) * sizeof(COLORMAP));

Then this problem disappears.

2. But then at the time of free()ing I get double free / corruption at core dump.

I am damn sure I am going wrong somewhere. I do not expect someone to correct my code and I just run it. Just hints would do.

Answer 1

I can see a couple of problems:

• ppColors is an array of pointers. Each element in the array is a COLORMAP* so you need to calculate the size to be malloced using numElements * sizeof(COLORMAP*). COLORMAP is just 3 chars so its likely that sizeof(COLORMAP*) > sizeof(COLORMAP). Your current allocation will be too small so you’ll end up writing beyond the end of the array; this has undefined effects but is likely to crash.
• use of width & height are reversed between allocations and loops so you’ll end up writing to unallocated memory at some point in your loops.