You have to know the root of the tree to do anything: void *tsearch(const void *key, void **rootp, int (*compar)(const vo

You have to know the root of the tree to do anything: void *tsearch(const void *key, void **rootp, int (*compar)(const void *, const void *)); void *tfind(const void *key, void *const *rootp, int (*compar)(const void *, const void *)); ... rootp points to a variable which points to the root of the tree. You can't operate on a tree unless you have a pointer to it, i.e., it's root.