Question

I have a struct PACKET which contains two int fields. How would I declare a dynamic array of PACKETs? I will be adding to the array an unknown number of times in the main…that is why I need it to be dynamically sized.

Answer 1

Encapsulate your dynamic array implementation with malloc/realloc in a .h/.c file pair for easier use.

static int length = 0;

PACKET *dynamic_array_put(PACKET *list, const PACKET new) {
  PACKET *ret_val = NULL;

  ret_val = realloc(list, sizeof(PACKET) * (length + 1));

  if (ret_val == NULL) {
    /* Not enough memory */
    return NULL;
  }

  memmove(list + length, &new, sizeof(PACKET));
  length++;

  return ret_val;
}

PACKET *dynamic_array_get(PACKET *list, int index) {
  PACKET *p = NULL;

  p = malloc(sizeof(PACKET));
  if (p == NULL) {
    /* Not enough memory */
    return NULL;
  }

  memmove(p, list + index, sizeof(PACKET));

  return p;
}

PACKET *dynamic_array_remove(PACKET *list, int index) {
  PACKET *ret_val = NULL;
  PACKET tmp;

  memmove(&tmp, list + index, sizeof(PACKET));
  memmove(list + index, list + index + 1, 
          sizeof(PACKET) * (list - index - 1));

  ret_val = realloc(list, sizeof(PACKET) * (list - 1));
  if (ret_val == NULL) {
    /* Not enough memory, restore! */
    memmove(list + index + 1, list + index, 
            sizeof(PACKET) * (list - index - 1));
    memmove(list + index, &tmp, sizeof(PACKET));
    return NULL;
  }

  length--;

  return ret_val;
}

If memory space is no issue, you can do the memory allocation in blocks, instead of at every addition of PACKETs and speed up the average addition.