How do I treat a pointer as a multi array?

Question

I have this loop which gives seg. fault.

  s->c = malloc(width * height * sizeof(double));
  if (s->c == NULL) { puts("malloc failed"); exit(1); }

  for (int n = 0; n < width; n++) {
    for (int m = 0; m < height; m++) {

      d = (&s->c)[m][n];

      printf("d %f\n", d);
      printf("m %i\n", m);
      printf("n %i\n", n);

    }
  }

Inside s->c is:

double* c;

When executed it just outputs:

d 27.000000
m 0
n 0

and then seg. fault.

It worked when I treated the s->c as a 1D array, but I would really like to treat it as a 2D array.

Is that possible, when the c pointer is in a struct?

If so, is (&s->c)[m][n] then the correct way to access the elements?

Sandra

Answer 1

The problem is that the compiler doesn't know the dimensions of your matrix.

When you have: double tab[m][n] you can access the element tab[row][col] as *(tab + (row * n) + col)

In your case you only have double *tab; that can be considered as the pointer to the element tab[0][0] with no information on the matrix dimensions and the compiler can't compute the right address.

You could compute the address yourself (for example using a macro) but would lose the nice tab[x][y] syntax.

I`m surprised it compiles. You should have received at least a warning about implicitly casting a double to a pointer.

Answer 2

I'm very surprised it even compiles. Apparently c is a double*, so (&s->c)[m] is the m'th double. Now, double doesn't have an operator[], so I don't see how the [n] part in (&s->c)[m][n] can be legal.

Presumably, you have declared c differently. There are different solutions: a pointer to a pointer, an pointer to an array of doubles, an array of pointers to doubles, etcetera. All might work, if the allocations match the declaration. In your case, the allocation will not match the declaration.