#include <search.h>

       void *tsearch(const void *key, void **rootp,
                       int (*compar)(const void *, const void *));

       void *tfind(const void *key, const void **rootp,
                       int (*compar)(const void *, const void *));

       void *tdelete(const void *key, void **rootp,
                       int (*compar)(const void *, const void *));

       void twalk(const void *root, void (*action)(const void *nodep,
                                          const VISIT which,
                                          const int depth));

       #define _GNU_SOURCE         /* See feature_test_macros(7) */
       #include <search.h>

       void tdestroy(void *root, void (*free_node)(void *nodep));

       tsearch(),  tfind(), twalk(), and tdelete() manage a binary tree.  They
       are generalized from Knuth (6.2.2) Algorithm T.   The  first  field  in
       each  node  of  the  tree  is a pointer to the corresponding data item.
       (The calling program must store the actual data.)  compar points  to  a
       comparison  routine,  which  takes  pointers  to  two items.  It should
       return an integer which is negative, zero, or  positive,  depending  on
       whether the first item is less than, equal to, or greater than the sec-

       tsearch() searches the tree for an item.  key points to the item to  be
       searched  for.   rootp points to a variable which points to the root of
       the tree.  If the tree is empty, then the variable that rootp points to
       should  be  set  to  NULL.   If  the  item  is  found in the tree, then
       tsearch() returns a pointer to it.  If it is not found, then  tsearch()
       adds it, and returns a pointer to the newly added item.

       tfind()  is  like tsearch(), except that if the item is not found, then
       tfind() returns NULL.

       tdelete() deletes an item from the tree.  Its arguments are the same as
       for tsearch().

       twalk() performs depth-first, left-to-right traversal of a binary tree.
       root points to the starting node for the traversal.  If  that  node  is
       not  the  root,  then  only  part of the tree will be visited.  twalk()
       calls the user function action each time a node is  visited  (that  is,
       three  times  for  an  internal node, and once for a leaf).  action, in
       turn, takes three arguments.  The first is a pointer to the node  being
       visited.   The second is an integer which takes on the values preorder,
       postorder, and endorder depending on whether this is the first, second,
       or  third visit to the internal node, or leaf if it is the single visit
       tree node the function free_node is called.  The pointer to the data is
       passed  as  the argument to the function.  If no such work is necessary
       free_node must point to a function doing nothing.

       tsearch() returns a pointer to a matching item in the tree, or  to  the
       newly  added  item, or NULL if there was insufficient memory to add the
       item.  tfind() returns a pointer to the item, or NULL if  no  match  is
       found.   If there are multiple elements that match the key, the element
       returned is unspecified.

       tdelete() returns a pointer to the parent of the item deleted, or  NULL
       if the item was not found.

       tsearch(), tfind(), and tdelete() also return NULL if rootp was NULL on

       SVr4, POSIX.1-2001.  The function tdestroy() is a GNU extension.

       twalk() takes a pointer to the root, while the other functions  take  a
       pointer to a variable which points to the root.

       twalk()  uses postorder to mean "after the left subtree, but before the
       right subtree".   Some  authorities  would  call  this  "inorder",  and
       reserve "postorder" to mean "after both subtrees".

       tdelete() frees the memory required for the node in the tree.  The user
       is responsible for freeing the memory for the corresponding data.

       The example program depends on the fact that twalk() makes  no  further
       reference  to  a  node  after  calling  the user function with argument
       "endorder" or "leaf".  This works with the GNU library  implementation,
       but is not in the System V documentation.

       The following program inserts twelve random numbers into a binary tree,
       where duplicate numbers are  collapsed,  then  prints  the  numbers  in

       #define _GNU_SOURCE     /* Expose declaration of tdestroy() */
       #include <search.h>
       #include <stdlib.h>
       #include <stdio.h>
       #include <time.h>

       void *root = NULL;

       void *
       xmalloc(unsigned n)
           void *p;
           p = malloc(n);
               return 1;
           return 0;

       action(const void *nodep, const VISIT which, const int depth)
           int *datap;

           switch (which) {
           case preorder:
           case postorder:
               datap = *(int **) nodep;
               printf("%6d\n", *datap);
           case endorder:
           case leaf:
               datap = *(int **) nodep;
               printf("%6d\n", *datap);

           int i, *ptr;
           void *val;

           for (i = 0; i < 12; i++) {
               ptr = (int *) xmalloc(sizeof(int));
               *ptr = rand() & 0xff;
               val = tsearch((void *) ptr, &root, compare);
               if (val == NULL)
               else if ((*(int **) val) != ptr)
           twalk(root, action);
           tdestroy(root, free);

       bsearch(3), hsearch(3), lsearch(3), qsort(3)

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