c - 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 :

int **a = malloc(rows * sizeof(int*));
for(int i = 0; i < rows; i++) 
   a[i] = malloc(cols * sizeof(int));

This maintains a unit stride between the elements in the rows, but this may not be the case between rows.

One solution is to convert from 2D to 1D, besides that, is there another way to do it?

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);
}