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       tsearch, tfind, tdelete, twalk, twalk_r, tdestroy - manage a binary search tree


       #include <search.h>

       typedef enum { preorder, postorder, endorder, leaf } VISIT;

       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 *));
       void *tdelete(const void *restrict key, void **restrict rootp,
                       int (*compar)(const void *, const void *));
       void twalk(const void *root,
                       void (*action)(const void *nodep, VISIT which,
                                      int depth));

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

       void twalk_r(const void *root,
                       void (*action)(const void *nodep, VISIT which,
                                      void *closure),
                       void *closure);
       void tdestroy(void *root, void (*free_node)(void *nodep));


       tsearch(),  tfind(),  twalk(),  and  tdelete()  manage  a  binary  search  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 second.

       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 the corresponding tree node.  (In other words,
       tsearch() returns a pointer to a pointer to the data item.)  If the  item  is  not  found,
       then tsearch() adds it, and returns a pointer to the corresponding tree node.

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

       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 argument  is  a  pointer  to  the  node  being  visited.   The
       structure of the node is unspecified, but it is possible to cast the pointer to a pointer-
       to-pointer-to-element in order  to  access  the  element  stored  within  the  node.   The
       application  must  not  modify  the  structure  pointed  to  by this argument.  The second
       argument is an integer which takes one of the  values  preorder,  postorder,  or  endorder
       depending  on  whether  this is the first, second, or third visit to the internal node, or
       the value leaf if this is the single visit to a leaf node.  (These symbols are defined  in
       <search.h>.)  The third argument is the depth of the node; the root node has depth zero.

       (More  commonly,  preorder,  postorder,  and  endorder are known as preorder, inorder, and
       postorder: before visiting the children, after the first and before the second, and  after
       visiting the children.  Thus, the choice of name postorder is rather confusing.)

       twalk_r()  is  similar to twalk(), but instead of the depth argument, the closure argument
       pointer is passed to each invocation of the action callback, unchanged.  This pointer  can
       be  used  to  pass information to and from the callback function in a thread-safe fashion,
       without resorting to global variables.

       tdestroy() removes the whole tree pointed to by root, freeing all resources  allocated  by
       the  tsearch() function.  For the data in each 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 node in the tree, or to the newly added node, or
       NULL if there was insufficient memory to add the item.  tfind() returns a pointer  to  the
       node,  or  NULL if no match is found.  If there are multiple items that match the key, the
       item whose node is returned is unspecified.

       tdelete() returns a pointer to the parent of the node deleted, or NULL if the item was not
       found.   If  the deleted node was the root node, tdelete() returns a dangling pointer that
       must not be accessed.

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


       twalk_r() is available in glibc since version 2.30.


       For an explanation of the terms used in this section, see attributes(7).

       │InterfaceAttributeValue              │
       │tsearch(), tfind(), tdelete()                       │ Thread safety │ MT-Safe race:rootp │
       │twalk()                                             │ Thread safety │ MT-Safe race:root  │
       │twalk_r()                                           │ Thread safety │ MT-Safe race:root  │
       │tdestroy()                                          │ Thread safety │ MT-Safe            │


       POSIX.1-2001,  POSIX.1-2008,  SVr4.   The  functions  tdestroy()  and  twalk_r()  are  GNU


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

       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 order.

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

       static void *root = NULL;

       static void *
       xmalloc(size_t n)
           void *p;
           p = malloc(n);
           if (p)
               return p;
           fprintf(stderr, "insufficient memory\n");

       static int
       compare(const void *pa, const void *pb)
           if (*(int *) pa < *(int *) pb)
               return -1;
           if (*(int *) pa > *(int *) pb)
               return 1;
           return 0;

       static void
       action(const void *nodep, VISIT which, 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 **val;

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


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


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