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.