How to dynamically allocate a contiguous block of memory for a 2D array

Question

If I allocate a 2D array like this int a[N][N]; it will allocate a contiguous block of memory.

But if I try to do it dynamically like this :

<

Answer 1

If your array dimensions are known at compile time:

#define ROWS ...
#define COLS ...

int (*arr)[COLS] = malloc(sizeof *arr * ROWS);
if (arr) 
{
  // do stuff with arr[i][j]
  free(arr);
}

If your array dimensions are not known at compile time, and you are using a C99 compiler or a C2011 compiler that supports variable length arrays:

size_t rows, cols;
// assign rows and cols
int (*arr)[cols] = malloc(sizeof *arr * rows);
if (arr)
{
  // do stuff with arr[i][j]
  free(arr);
}

If your array dimensions are not known at compile time, and you are not using a C99 compiler or a C2011 compiler that supports variable-length arrays:

size_t rows, cols;
// assign rows and cols
int *arr = malloc(sizeof *arr * rows * cols);
{
  // do stuff with arr[i * rows + j]
  free(arr);
}