Question

I am writing a C-program where I need 2D-arrays (dynamically allocated) with negative indices or where the index does not start at zero. So for an array[i][j] the row-index i should take values from e.g. 1 to 3 and the column-index j should take values from e.g. -1 to 9.

For this purpose I created the following program, here the variable columns_start is set to zero, so just the row-index is shifted and this works really fine.

But when I assign other values than zero to the variable columns_start, I get the message (from valgrind) that the command “free(array[i]);” is invalid.
So my questions are:

1. Why it is invalid to free the memory that I allocated just before?
2. How do I have to modify my program to shift the column-index?

Thank you for your help.

#include <stdio.h> 
#include <stdlib.h>

main()
{

int **array, **array2;
int rows_end, rows_start, columns_end, columns_start, i, j;

rows_start = 1;
rows_end = 3;

columns_start = 0;
columns_end = 9;

  array = malloc((rows_end-rows_start+1) * sizeof(int *));

  for(i = 0; i <= (rows_end-rows_start); i++) {
    array[i] = malloc((columns_end-columns_start+1) * sizeof(int));
  }

  array2 = array-rows_start;                          //shifting row-index

  for(i = rows_start; i <= rows_end; i++) {
    array2[i] = array[i-rows_start]-columns_start;    //shifting column-index
  }

  for(i = rows_start; i <= rows_end; i++) {
    for(j = columns_start; j <= columns_end; j++) {
      array2[i][j] = i+j;                             //writing stuff into array
      printf("%i %i %d\n",i, j, array2[i][j]);
    }
  }

  for(i = 0; i <= (rows_end-rows_start); i++) {
    free(array[i]);
  }

  free(array);

}

Answer 1

When you shift column indexes, you assign new values to original array of columns: in

array2[i] = array[i-rows_start]-columns_start;

array2[i] and array[i=rows_start] are the same memory cell as array2 is initialized with array-rows_start.

So deallocation of memory requires reverse shift. Try the following:

free(array[i] + columns_start);

IMHO, such modification of array indexes gives no benefit, while complicating program logic and leading to errors. Try to modify indexes on the fly in single loop.