How to free 2D array memory in C

Question

I am trying to allocate memory for 2D Array. But when I am trying to free up the memory it throws the segmentation fault. Kindly help me to know what I am doing wrong?

  int **arr1 = (int **) malloc (rows * columns * sizeof (int));

  //Array Access:
  for (i = 0; i < rows; i++)
    for (j = 0; j < columns; j++)
      *(arr1 + i * columns + j) = ++count;

  for (i = 0; i < rows; i++)
    for (j = 0; j < columns; j++)
      printf ("%d", *(arr1 + i * columns + j));

  for (i = 0; i < rows*columns; i++)
    free (arr1[i]);

Answer 1

It looks like you are trying to allocate an array of pointers to arrays. Your code segfaults because you free non-allocated memory. The proper way to allocate a "2D array" is more like this:

int **arr1 = malloc (rows * sizeof (int*));

for (int r = 0; r < rows; r++)
  arr1[r] = malloc(columns * sizeof(int));

...

for (int r = 0; r < rows; r++)
  free(arr1[r]);

free(arr1);

Answer 2

A simple rule of thumb is for every malloc there should be a free . You've allocated a block of memory that arr1 points to, so you should deallocate that one block of memory.

free(arr1);

You can't free a block of memory piecemeal. If you want to do that, you need to allocate each row individually, but using a single hunk of memory as a 2D array is also fine. Allocating each row is better if the size of each row is unknown. One big allocation is better if the row size is fixed.

---

In addition, arr1 should be a single pointer, not a double pointer. Compiler warnings should have warned you about this.

int *arr1 = malloc (rows * columns * sizeof(int));

A double pointer is a pointer to a pointer. It's not necessary to make a double pointer to pretend a pointer is a 2D matrix.

Answer 3

You can loop over each row and at each iteration, you can allocate space at creation phase and free at clearing phase it by yourself. Your code have several problems, I think that may be a better approach to write in a form like:

int **array = (int**) malloc( row * sizeof(int*)), i = 0, j;

for( ; i < row; i++){
    *(array + i)  = (int*) malloc(col * sizeof(int));
    for( j = 0; j < col; j++){
        *(*(array + i) + j) = i * row + j;
    }
}

This will create the 2d array with the values in increasing order.

While freeing:

for(i = 0 ; i < row; i++){
    free(*(array + i));
}
free(array);