5 * Sort linked list in place.
6 * - get_next_fn() returns the next element given an element of a linked list.
7 * - set_next_fn() takes two elements A and B, and makes B the "next" element
9 * - compare_fn() takes two elements A and B, and returns negative, 0, positive
10 * as the same sign as "subtracting" B from A.
12 void *llist_mergesort(void *list,
13 void *(*get_next_fn)(const void *),
14 void (*set_next_fn)(void *, void *),
15 int (*compare_fn)(const void *, const void *));