It really wants to see 'const void *' arguments.
/* If card_one < card_two, then return value will be negative
if they are equal, 0 will be returned,
if card_one > card_two, then return value will be positive */
-static int card_compare(card_t *card_one, card_t *card_two)
+static int card_compare(const void *one, const void *two)
{
+ const card_t *card_one = one;
+ const card_t *card_two = two;
return card_one->number - card_two->number;
}