C Struct with pointer to array

Question

I have the following code, and I'm a bit confused as to why I'm getting a segmentation fault.

typedef struct {
  int tag;
  int valid;
} Row;

typedef struct {
  int index;
  int num_rows;
  Row **rows;
} Set;

/* STRUCT CONSTRUCTORS */

// Returns a pointer to a new Sow.
// all fields of this row are NULL
Row* new_row() {
  Row* r = malloc(sizeof(Row));
  return r;
}

// Returns a pointer to a new Set.
// the set's index is the given index, and it has an array of
// rows of the given length.
Set* new_set( int index, int num_rows, int block_size ) {
  Set* s = malloc(sizeof(Set));
  s->index = index;
  s->num_rows = num_rows;

  Row* rows[num_rows];
  for (int i = 0; i < num_rows; i++) {
    Row* row_p = new_row();
    rows[i] = row_p;
  }
  s->rows = rows;

  return s;
}

/* PRINTING */

void print_row( Row* row ) {
  printf("<<T: %d, V: %d>>", row->tag, row->valid);
}

void print_set( Set* set ) {
  printf("[ INDEX %d :", set->index);


  for (int i = 0; i < set->num_rows; i++) {
    Row* row_p = set->rows[i];
    print_row(row_p);
  }

  printf(" ]\n");
}


int main(int argc, char const *argv[]) {

  Set* s = new_set(1, 4, 8);
  print_set(s);


  return 0;

}

Basically a Set has a pointer to a array of Rows. I'd think Row* row_p = set->rows[i]; would be the right way to get the row from a set, but I must be missing something.

Answer 1

You are allocating a local array of Row*s

  Row* rows[num_rows];
  for (int i = 0; i < num_rows; i++) {
    Row* row_p = new_row();
    rows[i] = row_p;
  }
  s->rows = rows;

and let the rows pointer of the Set point to that. The local array doesn't exist anymore after the function returned, so s->rows is then a dangling pointer. Memory that shall still be valid after the function returned must be allocated with malloc (or one of its cousins).

Answer 2

s->rows is assigned the address of the local variable rows in the function new_set(), which means s->rows is a dangling pointer when new_set() returns. Dynamically allocate an array of Row* to correct:

s->rows = malloc(num_rows * sizeof(Row*));
if (s->rows)
{
    /* for loop as is. */
}

Remember that s->rows, as well as its elements, must be free()d.