[C] Sort a singly linked list using Mergesort with a custom callback function

merge_sort.c

t_lst *merge(t_lst *left, t_lst *right, int (*fp) (t_lst *l, t_lst *r))
{
	t_lst *head;

	head = NULL;
	if (left == NULL)
		return (right);
	if (right == NULL) 
		return (left);
	if (fp(left, right)) //callback sort function 
	{
		head = left;
		head->next = merge(left->next, right, fp);
	}
	else
	{
		head = right;
		head->next = merge(left, right->next, fp);
	}
	return (head);
}

static t_lst *merge_sort(t_lst *head, int (*fp) (t_lst *l, t_lst *r)) 
{
	t_lst *left;
	t_lst *right;
	t_lst *slow;
	t_lst *fast;

	if (head == NULL || head->next == NULL)
		return (head);
 	slow = head;
	fast = head->next; 
  	while (fast && fast->next) //find the middle of the list
	{
		slow = slow->next;
    	fast = fast->next->next;
  	}
  	left = head; //pointer to the left side
	right = slow->next; //pointer to the right side
	slow->next = NULL; //set a break in the middle
  	left = merge_sort(left, fp);
	right = merge_sort(right, fp);
	return (merge(left, right, fp));
}

//example: list_head = merge_sort(list_head);