Is this new_node -> next = hashtable[hash]; right? Assuming hashtable is an array of ints, and new_node-> next is a node pointer, then something's amiss.

Don't know if this is root of the problem, but it jumped off the page.

Is this right new_node -> next = hashtable[hash]; ? new_node is pointing to the head of the list and then hashtable[hash] -> next = new_node; head of the list is pointing to new_node. As @Blauelf said, pointer loop.