In your insert_node function, while your iterating to find the last node, you should check whether the next_node is NULL, not current_node is NULL.

Let's say, we had two nodes

  1   ->   2   ->   NULL
head

When you start iterating, using

while(current != NULL) 
   current = current -> next;

It will eventually reach the node 2. now this is the last node, you should stop here, but according to your while loop(current != NULL), it will move onto NULL and then break out of the loop.

So current = NULL at the moment. No when you do current -> next = newNode, you're de-referencing NULL, resulting in seg-fault.

So only modification would be

while(current !=NULL && current -> next != NULL) //Check whether the list is empty, if not fetch me the last node

In your insertNode function, while your iterating to find the last node, you should check whether the next_node is NULL, not current_node is NULL.

Let's say, we had two nodes

  1   ->   2   ->   NULL
head

When you start iterating, using

while(current != NULL) 
   current = current -> next;

It will eventually reach the node 2. Now this is the last node, you should stop here, but according to your while loop(current != NULL), it will move onto NULL and then break out of the loop.

So current = NULL at the moment. When you do current -> next = newNode, you're de-referencing NULL, resulting in seg-fault.

So only modification would be

while(current -> next != NULL) //Fetch me the last node

But there is till one important thing to do. You're not checking whether list is empty or not inside insertNode function.

if (head == NULL) {
    node_t *newNode = getNode(val);
    head = newNode;
    return;
}

Edit: Looks like you're assigning a node to head inside main function. The above check is not necessary